Probably something about the size of a half whiskey barrel planter. Larger is better. You may have overwintering issues if you're in the colder states.

I've done well with sweet cherry, nectarine, apricot, and much more in 12-15 gal pots. Peach will work the same. Just make sure your mix drains very well.

This picture gives a good idea of tree size in a 15 gal pot.

{{gwi:76099}}

What the heck is that behind the tree? A Japanese shower?

I shouldn't answer for someone else, apologies in advance FN, that is what is known in the greenhouse industry as "pad and fan evaporative cooling" -- what you see on this end of the greenhouse are cellulose pads that are kept moistened by a constant, light drizzle of water from above. On the other end of the greenhouse, somewhere out of the photo, is a big fan that moves the air. As the air moves over the moist pad, water evaporates, cooling the air quite a bit -- exactly how much cooling is achieved is a function of factors such as the air flow and more importantly, the dew point/humidity of the air. Pad and fan cooling is very effective in dry areas, somewhat less so in areas that are very humid/high dew point. I think it's quite common to get a reduction in temperature of about 20 degrees F.