Hey guys I have a chance to order a fred battery from work, what size are in our trucks. It's a 1956 f100 with a 292

 

Do you mean physical dimension, or AH capacity?

 

The parts book list 3 different size batteries for 56..
Ford 55 amp - 12 1/2L x 5 1/2W x 8"H SAE group 4NFA
Ford 78 amp - 14 1/4L x 5 1/2W x 8"H SAE group 5NA
Non-Ford 40 amp - 10 1/4L x 6 3/4W x 7 7/8"H SAE group 2SM

Of course these group numbers no longer exist and nothing in the current BCI group sizes are the same.

If your battery is in the original location, under the passenger side floor, I'm not sure if that 'non-Ford' size would fit. Unless somebody comes along with the correct answer my suggestion is to measure the battery holder and the hold down to see what size fits, and which side the positive post belongs on and go to this site to determine what BCI group will best fit your needs.
https://www.batteryweb.com/bci.cfm

Being in IL. I would lean toward the higher CCA rating unless you never start the truck in cold weather.

 

Thanks Harley, I don't have the battery tray anyhow