15 should be fine. we used 7-12 gallon 2 years ago and averaged 3-4 from the 7's, and 5-7 from the 12's. line the bottoms of the bags with rocks if they're available, helps with both drainage and stability
nah i like the grow bags there awesome just planted 1 in a 15 gallon grow bag and its perfect and easy no digging im tired of digging dug 6 35 gallon holes for my grandaddy purp clones plus the bags were only a buck a piece!