The sizes I've been accustomed to working with are:
Lap quilt: 54" X 72"
Twin quilt: 54" X 90"
Double quilt: 72" X 90
Queen: 90" X 108"
King: 108" X 108"
Where I'm originally from, they didn't have "California King".
In terms of computing the size of the quilt, I think it would be more helpful to look at the size of the outside perimeter of the mattress, which doesn't change, and then take into account the depth of the mattress, which changes constantly. I think that's why there are so many different versions of quilt sizes. That mattress keeps getting thicker and thicker. I just picked up a set of sheets that have a 25" depth to them in order to fit our new "pillow top" mattress which needs that much depth. And there were sheets there meant to fit deeper mattresses! A mattress 25 inches deep was unheard of when I was growing up, and when Great Grandma made a "feather pillow top" for me, it went on top of the sheets.
I digress. Here are the unchanging measurements of standard mattresses:
King: 76" X 80"
California King: 72" X 84"
Queen: 60" X 80"
Full XL: 54" X 80"
Full/Double: 54" X 75"
Twin XL (Often found in Dorm rooms): 38" X 80"
Twin: 38" X 75"
Crib: 27.5" X 52"
Now, if you know the size of the bed, it has to fit the parameters above. The question mark is: how deep is the mattress? If you know the person you are making the quilt for, you can always ask them to measure, but I know this is impractical in a number of cases. I tend to err on the side of making them a little larger. I know my mattress is 25" deep, so I'll make sure I've got at least 35" of drape over the size of the mattress. I figure that covers all sins. If I've seen the bed, I'll have a fair idea of how much drape I need to make.
But without bashing this idea to death, I'm sure you've gotten my point.
Have fun quilting!