If possible, I measure the bed the quilt is for, and find out if the person likes to do the pillow tuck or not. That's really the only way to get a guaranteed fit. Otherwise I usually look up the dimensions for good quality comforters online for whatever the bed size is, and see if I can find a sort of "average" size to go with.
I made a double for my MIL and had to measure her bed carefully - it's an antique bed where the head and footboard and side boards really sort of encase the mattress (so no room for extra fabric) and she doesn't use box springs and doesn't do a pillow tuck, so her quilt ended up much smaller than I would have made for someone else who uses a more modern style of bed. It really seemed TOO small when I finished it, but she says it fits great. *whew*