A 0.75 cubic ft softener will be sufficient for your needs. Since you have iron in your water, you will need to set it to regenerate every four days, rather than regenerating based on water usage.

You may wish to consider separate iron removal prior to the softener. That would allow you to go three weeks between regenerations and reduce your salt usage from 45 to 10 lb per month. The trade-off is more expense up front.

Thanks aliceinwonderland_id. The 0.75 was what I was guessing at but wanted to be sure.

What kind of iron removal unit should I be looking at? I'm not very familiar with them. I guess I will need to weigh the upfront cost of that kind of unit vs. the cost and hassle hauling salt home. What kind of maintenance is needed for an iron removal system?

Thanks

General iron removal information:

Softener:
A softener with specialty resin is capable of removing dissolved
iron, up to 7 ppm (mg/L) but asking a softener to remove iron at
those upper limits is really pushing it so I do not recommend this
method. In addition, a softener will become fouled with particulate
iron. A softener will not remove sulfur compounds.

Oxidation/Filtration:
An oxidizer such as ozone, air, or chlorine may be used to react with
the iron and force it to become particulate iron that can then be
removed via filtration. This will also remove most sulfur
compounds. A typical setup would involve the oxidizing unit, a
holding tank, then a media filter. This type of system works quite
well, but takes some expertise in sizing - you would want a water
treatment pro that you trust to help you with this option.

Oxidizing
filtration media: This type of treatment consists of a sealed tank
filled with one of several media. Water passes through and is
oxidized and filtered by the media. Most will remove sulfur compounds
as well as iron. These are relatively easy to operate and what I
would recommend for the average homeowner. There are several
different media that can be used:

1. Manganese
   greensand: water runs through for treatment. The media must be
   regenerated with potassium permanganate. Care must be taken with
   dealing with potassium permanganate as it readily dies organic
   material, such as your skin, a purple-brown color. Some people are
   quite comfortable dealing with the chemical; others are not.
2. Synthetic
   greensand: This is essentially the same as option (1) but consists of
   a coating of greensand on a silica sand core so does not require as
   much backwash flow. Service flow rate is 2 - 5 gpm/sqft. Backwash
   flow rate is 12 gpm/sqft.
3. Birm:
   This media acts as a catalyst to force oxidation of iron. While it
   does not need to be regenerated, it does need fairly high dissolved
   oxygen in the water. If your water does not have adequate dissolved
   oxygen (and it probably doesn't since it is well water), air
   injection would be necessary prior to the birm. Additionally, birm
   requires a minimum pH of 6.8. Service flow rate is 3.5 - 5.0 gpm per
   sqft. Backwash flow rate is 11 - 20 gpm, depending upon water
   temperatures and desired bed expansion.
4. Pyrolox:
   an ore that oxidizes then filters the iron out. It does not need
   regeneration, but needs to be backwashed (to rinse out the iron) at a
   high rate. pH range is 6.5 - 9.0. This type of filter works very
   well, but backwash is critical. Service flow rate is 5 gpm/sqft.
   Backwash is 25-30 gpm/sqft. Backwash daily.
5. Terminox:
   Similar to Pyrolox, but a proprietary formula . It does not require
   as much backwash flow rate and is more resistance to a low pH. The
   particulars are only available from the company that sells it - this
   means you have to rely entirely on the vendor for proper sizing.
   Backwash daily.
6. Filox:
   Also similar to Pyrolox. pH range 5.0 - 9.0. Backwash flow 12-15
   gpm/sqft. Service flow 6 gpm/sqft. It must be backwashed daily.
7. Pro-OX:
   Essentially the same as the previous three. Requires 12-15 gpm/sqft
   backwash. As with Terminox, specs are lacking information so you'll
   have no way of knowing if the unit you get is sized appropriately.
8. Katalox-light:
   Similar in action to the previous four, but is comprised of zeolite,
   manganese dioxide and hydrated lime. It should be designed for 6 -
   12 gpm/sqft service flow and 8 - 10 gpm/sqft backwash. For well
   water, it's a good option as it requires lower backwash.
9. In
   all cases, media ought to be replace somewhere between 5 - 10 years.
   If any vendor tells you their media lasts forever, walk away - they
   aren't trustworthy.

Thanks for taking the time providing that wealth of info aliceinwonderland_id.

It would seem the simplest might be to just let the softener deal with the iron, especially since the iron level is fairly low. My only concern with that is the house will be on a septic system and I worry about pumping that much salt into the system. If I were to go to a longer period between regenerations it would foul the media more quickly, correct? Is there any way to revive the media if it gets loaded with iron or is replacement the only option?

Thanks again.

There is a process that MAY revive the resin to an extent, but it is pretty involved and not likely to bring it back to any better than 50% of manufactured capacity. I would never recommend going longer between regenerations with iron-laden water because you would be unhappy within two years at most.

The
problem with iron-laden water in a softener is that softener resin
has a great affinity for iron and does not release it easily. Iron
will slowly foul the softener resin from the inside out. If you want to get maximum life out of your resin, extra measure should be taken, in addition to regenerating every four days:

1. The
   easiest option may be to simply use Iron-Remover salt.
2. Alternatively,
   you can use Super Iron Out. You may choose to layer this in with
   your salt. If you choose this option, use 1/4 C per 40 lb bag of
   salt. Super
   Iron Out is more effective if used monthly with the following
   procedure: 1) Ensure
   you have a strong brine in your brine tank - at least 24 hours since
   the last regen. 2) Dissolve
   1 C Iron Out in cool water and pour it into the brine well inside
   your brine tank. If you don't have a brine well, for some reason,
   pour it down the side of the tank rather than through the salt.
3. Place
   your softener into manual regen and stay right there. 3) When
   backwash is complete, the softener will move to the brine/slow rinse
   mode. Pay attention to the way it sounds. After 10 minutes or so
   the sound will change when there is no more brine to draw. At this
   point, we want the Iron Out to sit in the softener for an hour. To
   do that, push the button to advance to the next regen step, then the
   next until your softener is back in service mode. Do this as quickly
   as you can. 4) Switch
   your valves so your softener is bypassed and wait one hour minimum. 5) Place
   your valves back in service mode and manually start a softener regen. Once
   the regen is complete you're good to go for another month.
4. A
   liquid iron removal system such as Res Up can be installed in your
   brine tank to feed phosphoric or citric acid to the brine to aid with iron removal.

My preferred methods are 1 or
4 because they employ phosphoric acid, which is a much safer chemical
than those in Super Iron Out (sodium hydrosulfite and sodium
metabisulfite). The choice is yours - pick a method and follow it
religiously and your softener will give you many years of reliable
service.