What size softboxes should I buy for full body portraits?

That depends on the effect you want.
The smaller your light source in relation to your subject, the harder the light, the bigger... the softer.
Now I read that you are working in a small space... then you should definitely take into account that the light may also reflect on the walls, which makes your light softer. If you are working in small spaces, it is definitely advisable to have softboxes that can be equipped with grids, making them more directional.