|Image Source: Pacific Star Products|
Once you know the mattress size, you can decide how long you want the drop to be (i.e., how much you want it to hang over the sides), or if you want a pillow tuck, etc. to begin determining the finished size of your desired quilt.
Now, you can look at your perfect pattern and know if it will fit your bed! And if it doesn't, at least you now have a point of reference to figure out how many more blocks and /or borders to add to get it to the correct size.