The biggest pot that you have room for inside and can move. If you can move it, try a cognac barrel (79 gallons).

If you are going to keep this tree in a container, a good MINIMUM is between 15-18 gallons. Anything smaller and you will have to deal with too frequent waterings, heated growing medium, stressed trees, restricted root systems, and, frequent root pruning/re-potting, etc.

Large capacity containers will become very heavy, especially when the growing medium is wet, and they are difficult to move around unless you use a hand truck, or place the containers on a dolly. Home Depot sells rectangular, 18 gallon storage tubs which can make serviceable containers if you drill holes along the bottom-sides for drainage. The tubs cost less than $10.00, and you can re-use them after doing a root pruning, so your tree can be replanted into this tub for a long time.

18 gallons of soil is plenty of medium in which to grow a fig provided you give it correct nutrients, water, and try to keep the direct sunlight off the container. Try to avoid heat-stressing your tree. Keep the container (root-zone) shaded and cool, but keep the leaves, and figs in full sun. Just make sure you use a quick-draining growing mix, like the 5-1-1, and throw in a good handful of granulated limestone to keep the mix sweet.

Growing in containers is not problem-free, and the fig trees will require attention. However....the trees can easily be moved/rolled into a cool storage area for the winter months. This dual, cool storage, and protection from freezing, will almost guaranty that your trees will survive. You will also get a jump on the spring, growing season, by shuffling your trees. Trees go out when the days are warm and sunny, and then back inside at night when temps. plunge into the danger zone. You can get a good months heads start by doing this, and extend your fig season. With in-ground trees, not so much.

I've been growing in containers since 2007, and I find that the 18 gallon containers are about the largest that you want to go if you need to move these containerized trees around. In Zone 8a, I guess you can get some pretty cold weather and freezes, so container mobility might be a factor to consider. Dollies help.

Good luck and have a good season.

Frank