For RKN, or Root Knot Nematode, there are several preventative measures for in ground figs. I have gathered this info over time and from many internet sources. If you're in sandy soil near the southeastern US coast, you probably have them. And they love fig roots. This is how I plant my trees at my coastal NC property - this is my first year, but all of my trees did well through their first season.
First, amend the soil with a lot of organic matter, e.g. compost or potting mix (not soil).
Second, cut the bottom off a 5 gallon bucket, plant the tree in the bucket and the bucket in the ground with about an inch above the soil line. Use high quality potting mix or 5-1-1 mix, do NOT add your local soil!
Third, mulch with organic mulch.
Apparently RKN do not travel deep into the soil (they need light), and they don't like being around high organic matter soils. So your trees roots will emerge from the bottom of the bucket and not be bothered by them. Keeping it mulched will keep new organic matter going into the soil creating a buffer zone for your tree.
I believe you can always keep your fig tree as small as you like with pruning.