Hi - to find the side of a square when you know the diagonal, you divide by the square root of 2 which is 1.41421 35623. For your 22" square this comes to about 15 1/2". Hope this helps (no, I'm not a math genius - ain't google great! :wink: )
I agree with you that the main images in your quilt would look nice on the bed top, then maybe with a plain border or more simple pieced border to hang over the sides or tuck in, as he wishes.
What a wonderful project! :D