In choosing a garden size there are a few factors to consider. The more time spent assessing and planning the more likely you are to build a garden you enjoy.

As a general rule the better the soil (fertility and tilth) the more closely you can space plants without sacrificing yield. If your desire is to plant in single rows which 'wastes' a lot of garden space then the garden area needs to be larger than if you intend to use wide rows of crops planted closer together.

How will you weed and water the garden. Every bare spot of ground will grow many weeds. Planning now to limit weeds will save you hours and hours of sweaty work during the growing season. You can either limit the bare ground areas with close crop spacing or employ the use of a mulch to keep the non growing spaces covered(or both).

Watering the garden can be done several ways and perhaps the easiest, cheapest but most trouble prone method is using an overhead sprinkler. The sprinkler will waste a lot of water particularly on windy days and it will result in plant foliage being wet more often than it would otherwise which can increase the severity of fungal pathogens on the crops. Using an overhead sprinkler will also wet the entire garden including the areas not supporting crops (pathways). Mulch or no mulch areas being watered will grow more weeds than those not watered.

Hand watering with a hose can limit wasted water and also eliminate much of the wet foliage, but it is labor intensive and you need to get a feel for how much water the plants need. Often when standing with a hose in my hand I have to discipline myself to water one area long enough before moving on. It is also no fun when the mosquitos and other pests are out.

Soaker hoses are a good investment as they waste next to no water and put the water right where it is needed. They can be attached to a timer for added convenience. They do cost some money and require some upfront labor and they do not last forever. In areas with hard water they probably should be replaced each year.

Also don't overlook things like raised beds. This will allow you to focus all your amending into fairly limited areas and the rest of the area can then be mulched with something comfortable to walk on.

My own gardening began much like your appears to be. I tilled up a 24x24 area. Added organic matter and tilled again and then started gardening. Early in the season I kept up with the weeds, but later on when it was hot, muggy and buggy I didn't keep up with weeding for several days and really never did get control over them again.

I added the use of a mulch in subsequent years and this helped, but added to my early season labor in trucking in the mulch and in many cases the mulch didn't last the entire season so had to be topped off to help with the weeds.

Eventually I built raised beds in the area and planted grass seed between (be careful to not use a creeping type grass). Best thing I ever did. My usable yield is comparable due to less wasted space and being able to focus on the soil in those beds being 'perfect'. Next to no weeding as well.

Just some thoughts for you, hope something is of use.

git yer soil tested - although organic compost is good, there may be deficinecies you need to address, now is the time deal with acidity/alkalinity issues as these take time to resolve.

20x20 is a big garden, a good place to start, you can always increase space next year, I increase min every year bit by bit.

rye may not have enough time to germinate and grow enough at this point in time for Maine, but you could still get a few bulbs of garlic in the ground..

It has been a long time since I was beginning but one thing you should know, Peter, is that each year is a new one. Jefferson said something about being an old man but a young gardener.

About the rye, I believe that it is too late in the year altho' you may get some growth. I'm probably at the same latitude as you are and my soil was 42°F, 2 weeks ago. I understand that rye can germinate down to 33°F but it won't really grow at those temperatures. Purdue University says, "The best time to seed winter rye is from August 15 to September 10 in northern Minnesota and Wisconsin, and from September 5 to September 30 in southern areas of the two states." When I've grown it, I have really liked to have the seed in the ground by August 1st - it made HUGE growth over the Winter when planted that early.

The size of the garden in part depends on how many people you are hoping to provide with fresh vegetables. The garden for a family of 6 will be different than one for a couple. Often, I see Cooperative Extension suggest a garden of about 800 square feet for a family of 4 but that's fairly good size for a lone gardener with only a few hours each week to tend.

The amount of compost you have sounds good for the garden size. It'll never seem that you have enuf compost and spreading it thinner than a couple of inches will have very little benefit.

If this is ground that has grown weeds in the past, you'll likely have lots of weed seed. One way to deal with that is to delay the Spring planting of your vegetables so that you can cultivate the ground repeatedly.

Certainly, you don't need to plant vegetable seed until you see weed seed germinating and beginning to grow. Those tiny weeds are easily killed even with a rake at that stage. Trying to protect your vegetable seedlings will getting the weeds out later is real time-consuming and not much fun. Often it is better to kill the weeds first, allow the soil temperature to get closer to ideal for quick veggie seed germination and then let your garden plants take over some of the responsibility of out-competing the weeds.

I think what Virginia Tech has to say about Intensive Gardening makes a lot of sense and may fit with your requirements. But, this pdf guide from Cornell may be just where you need to start out.

Best of luck and happy gardening.

Steve

One thing to consider about winter rye. If you have a late and wet spring, it's hard to get a handle on the rye. It can easily get out of control and hard to till under.

I'm planting winter rye tomorrow - seed is cheap so why not give it a try No frosts yet in my sister's garden in Bath so plenty of time for rye to germinate and grow. Depends on what zone you are in - 4 might be iffy but 5 is OK.
If it gets too high in spring, I just cut and use it for mulch - stubble and roots not hard to turn under.And yes get a soil test now - winter is a good time to lime.

I just pulled the rye up. I suppose with clay soil it could be difficult but with my "gravel mix" it's easy.

I had more trouble with it when it was planted late and I couldn't get a hold of the rye to pull it out of the ground. Then I HAD to turn it under to kill it.

I've planted Austrian field peas late before and had 99% winter-kill. I looked on it as just fertilizing with the pea seed. And, at the high cost of organic fertilizer - that was comparable.

Steve

I'm a strong proponent of permanent, no till beds (not raised), permanently mulched and with mulch between them. Easy to establish, easy to maintain, virtually no weeds, and terrific yield.