i use the 2 & 3 gals for flowering depending on plant size. i can yield out of a 3 gal fabric pot what friends get out of 5 gal hard pots. and im in colorado where its really dry and they help raise humidity in the tent. you really shouldnt see too many roots protruding from the sides unless you have pots touching each other which defeats the purpose of using fabric pots altogether. ideally you'd have them on some sort of rack or shelving that allows air to flow underneath them as well. i built tables for my tent that have wire shelving on the top, the kind you'd find in a closet or something.
start with good soil, though. fabric pots have tendency to dry out quicker especially if elevated, which means more frequent watering which can lead to the less experienced burning their plants or salts building up and locking out certain nutes. sometimes these are mistaken for deficiencies and again the plants get burned. if you start with good soil you shouldnt need to do much but add clean water and root builders until flower when you begin supplementing P.