what size container do i need to grow apples? thanks

Depends on the size of the tree you have. If your talking about growing an apple tree from seed, forget it. Apples produce very poor tasting fruit when started from seed. Growing apples from seed is never done. It is generally stated that the chance of getting a good tasting apple from a seed grown tree is 1 in 100,000. - Millet

You can get dwarves or cordon apples, and grow them in probably 5-10 gallon pots. Stark Bros and others carry dwarves.

Ive seen crab and standard apple bonzai, but those would have lower yield.

thanks