JDS
There are some threads that discuss pot volume, etc. that were previously posted on this forum, if I remember correctly.
Home Depot sells large storage tubs, and these can be converted to a planting container very easily by drilling, or melting, holes into the bottom SIDES of the tub...not into the bottom, but only the sides, of the container. Why? When the roots grow out of the drainage holes drilled into the sides, you can easily spade-trim/slice the roots to remove the tub...but if the holes are drilled into the bottom, and the roots grow into the soil, the tub will be firmly anchored into the soil, and James will be very annoyed when he tries to lift the tub for whatever reasons. Drill side holes...it prevents hernias.
Home Depot, and other such stores usually sell a wide range of tubs that are very easily converted into growing containers for figs. Buy a dolly, and put your heavy tubs on it, and roll your trees around. Doing this, also prevents hernias. Volume of the storage tubs is usually on the label, and shapes vary even with the same capacity in volume. The Home Depot storage tubs are rectangular, and have handles for easy moving, and lifting. You can grow your trees very easily, for years, in one of these 18 gallon containers....and, they're cheap too. You will need to root-prune, and plant the trees in new medium every 3-5 years, and the tress can be replanted back into the same containers. If planted in ground, disregard this whole discussion.
Good luck.
Frank