I'm more willing to believe the one wanting to go with a smaller size (but a load calc needs to be done to determine size and other modifications to the ductwork need to be made before any comfort can be guaranteed with the new system). I have a 3.5 ton system at my house, and there is one 14x8 trunk and one 20x8 trunk coming off the main plenum, which is 20x20. So, 8x14 doesn't sound nearly adequate in my opinion.

According to a sizing chart available from HVAC-Talk, a 14 x 8 duct is good for a maximum of 490 CFM. That is barely adequate for a one ton system.

I think you need both a load calc and new ducts. I would question any contractor who would install even a 2.5 ton system using that existing ductwork.

Here is a link that might be useful: Duct Sizing Chart

Thank you for the replies, I apologize, I measured the return vent by mistake.
The correct measurements
The duct directly at the furnace measures 34 x 8 inches, three feet out from there, it measures 20 x 8, then it drops to the 14 x 8 three feet further. Still too small?

Are there any branches between the 34x8 and the 14x8 points? If not, it is still too small. If yes, please describe them.

If it isn't choked off, as it seems it is from your description, the 34x8 duct would be adequate for a 3 1/2 system at best. Other factors might reduce that figure.

BTW, your return duct(s) needs to as large or larger than the supply duct. In other words, you have to be able to freely return at least as much air to the system as it is capable of supplying.

Thank you for all the info! This is all new to me.
There are three 22 inch diameter branches coming off of the larger duct before it reaches the 14 x 8 inch section, so it doesn't appear to be choked off.
For the return, one 14 x 8 inch trunk leads to a five foot 44inch diameter piece that connects to the furnace. There is an additional 22inch diameter branch from another return connecting here as well.

Provided the return grills themselves are adequate, it sounds like you have adequate return capacity. Your 34x8 supply duct seems to be the limiter at 3 1/2 tons.

However, there could be other factors which may reduce that number. The contractor suggesting 2 1/2 tons very well may be seeing something that isn't apparent in this thread. Contractors don't like to down sell, unless there is a pretty good reason.

I still think you need to ask them to do a load calc.

Thanks again. The price for the 3.5 ton Goodman was $3500. For the contractor who would go with the smaller 2.5 ton Rheem was $3800. I am going to call a third and make sure he does a true load calc.