If all of the mattress sizes are twin, I'd make it a twin-sized quilt. Maybe I'm not understanding your question?
I personally like a drop to come down to at least the bottom edge of the mattress on a twin. I prefer kings to be even longer, because you usually have 2 lumps (humans) who draw the quilt up. There's nothing worse than a quilt that doesn't quite cover everything and you wake up with an ice-cold flank in the middle of the night. 😉