a number of 'ranges' are used but basically:
baby- 36"-45" x 36"-52"
crib- 30"-36" x 46"-50"
bunk 66"x 80"
wheelchair lap- 38"x48"
lap- 52-68" x 52-78"
twin- 64-72" x 86-90"
full 70-80" x 88-100"
sizes are often (customized) according to the size of the person who will be using it- such as - my oldest son is 6'4" so i make his (snuggle quilts) really long!
my middle neice is 4' 9" so i make her's shorter than (normal)
you might look at batting packages too to help determine sizes