the pot size you would buy depends on the size of the rootball of the tree. once you determine that, pot size will be easy.

If you get a tiny tree with a small rootball, or a bare-root tree, and put it in a giant container, you will have some trouble keeping the containers wet. In general, you should get a container appropriately sized for your plants and pot them up as needed. If you buy a tree in a 1g, you can pot it up right away into a 3g--maybe a 5g if you are willing to water more until the roots spread out into the surrounding potting media. Cody's advice is right: choose the pot based on the size of the rootball. Otherwise it's really hard to answer your question which is sort of like asking 'how long is a piece of string?'. there are too many factors.

I have a container based tree farm. I often plant seedlings in 1 liter cell styroblocks to start with. For a blue spruce they can happily stay here for 1-2 years (I'm in Canada -- short growing sesaon.) Some I plant directly into #1 pots.

The 1 liter trees can go into a #2 pot. The #1 trees go to a #3 or #5.

In general, deciduous trees grow faster than conifers. A 4 year old blue spruce (a slow conifer) is 2.5 feet tall. A 4 year old quaking aspen (fast deciduous) is several times as large.

Much too depends on your fertilizer regime. I've seen pix of caliper sized trees in growbags that were only 2-3 years old and 2 inches across. Mind you that was in central B.C. with twice the growing season.

Sherwood Botsford
Sherwood's Forests Tree Farm
"Trees for Rural Living"
http://sherwoods-forests.com
sfinfo@sherwoods-forests.com
50042 Range Road 31, Warburg, AB
(780) 848 2548