It entirely depends on the donor site you choose. You are restricted to whatever size they can give you based on that. The big limitation is the forearm - you're confined to the length of your forearm, minus a few inches where the harvest the leash. I'm not sure what the restrictions are as far as size for the thigh or back. Those are the three primary donor sites being used currently. Abdominal is still used but less commonly and by only a few surgeons. The largest I have ever seen is 7", and his came from the thigh.
In general, smaller is recommended for a few reasons:
(1) You're that size all the time. Unlike cis guys who get bigger when erect, you are permanently that big. It can be inconvenient and it would take a while to get used to how to place yourself in certain clothes.
(2) Blood flow. It is much easier to re-establish blood supply in 4-5" than in 7+". It's important not only for the health of the phallus as a whole, but also the neo-urethra if you opt for urethral lengthening.
(3) Gravity/weight/support. Like with blood flow, it is much easier to support something that is 4-5" and weighs several ounces less than something that is 7+".
I don't think size impacts surgery recovery. Obviously you would have a larger donor site to care for post-op, and you would possibly be at a greater risk for necrosis if the blood supply doesn't re-establish well.