Go to one of those "constant sales" mattress shops and prepare to be stunned at all the different sizes of mattresses. Plus, you can have a mattress made in whatever size you want. My sister had one made for her antique, not standard bed and loves it.
And then there's bunk beds and "big boy" beds that are not twin sized, etc.
30 or so years ago when DH and I got a new mattress, there was a King size, a California King size, an Eastern King size, etc. It may be possible that since then they've standardized all those sizes, but not likely.
Suggestion...measure the bed you think this will be on for a long time, then measure from the top of it to the floor and decide how far down it should come.
As for me, I want quilts that hang down to the floor to cover the drawers under my high wooden bed.