What size of bags?

I am going to be putting 25 or 30 "trainwreck" outdoors around the 1st of June. They will be 12-18 inches tall when they get put out.

If I use grow bags instead of digging the holes, what size bag would be sufficient?

if your looking for maximum growth, go for huge bags. Im using 7gallon bags which is pretty small for an outdoor grow, but whatever i dont really want them to get that big cause they are in a suburban area