Given that the tree grew from seed it could get rather large. Seedlings will usually produce a full sized tree - 30 to 40 foot tall. You will need to find an eventual site that is clear 15 to 20 feet in all directions from the trunk. Your tree will not be a Granny Smith but a cross between the Granny and whatever the pollen source was. In commercial orchards that could be a crabapple. You have a very slim chance that your seedling will produce decent fruit. I plant a couple hundred seedling each year just for fun and I've only had 2 or 3 trees that were OK for fresh eating fruit. The rest became rootstocks. If you plan to go forward with your seedling I would recommend a 5 gallon pot for the first couple years. Mix native soil with a potting mix 50-50. Good luck - seedlings are fun to grow, you never know what you'll get.

Tammypie;

You could also save some time and just get a Granny Smith apple bare root from a retail nursery this spring, which are usually grafted on dwarfing rootstock to remain managable in your yard. Granny Smith is an excellent variety for your Orange County location and will bear regularly and heavily with little trouble. The apples are superior to what you get in the store; if it looks a little lonely out there, Fuji is a good companion for it.

Applenut