Look for a high percentage (>50%) female not male. Yes males stay smaller but my 100% SD male is an eight foot jerk that threat charges and likes to back up to you and spray you with pee/musk, and he does it on purpose while looking at you.
My 50% SD adult female is about 11 feet, absolutely chill and easily handled, not flighty, etc. She's still a big snake but not massive like a mainland will get.
Spend the money to get one from a reputable breeder. There are a lot of non-SD or low-percentage SD non-morphs advertised as high percentage because they command a higher price, and by the time you figure out you were sold a pure mainland that is going to top out at 18-20 feet the seller is long gone.