A toddler bed has a crib mattress - 27-ish by 51-ish. I'm being commissioned to make a quilt for a friend's dd's toddler bed, she requested 45"x65".
For what its worth, if this is a snuggle blanket, my 5yo dd is 43" tall and on the tall side of her class, so 45-50" square would not be too small for a 3yo to use for a couple of years.