Double bed is about right for a queen mattress. For a 4 wide that is a double and a double would be too wide for you. Doug makes his a little smaller, I think, just barely to the edge of the bed on the sides, but I wanted to tuck in some and have more to cover with, as you said , so mine is about 8" wider than the bed on either side. Any more and you just have bunched up fabric. Mine is a little longer on the bottom, so I can actually tuck it UNDER the mattress and it stays in place a little better. But it is quite a job to get it under there in the first place! If I were to do it again, I would probably make the sides 6" wider than the bed. 8 is a tad too much on the sides.
Oh and don't bother with a fancy border. I spent a lot of time on a nice pieced border that I love, but you can't see it cause it's tucked in or covered by pillows!
If your quilt covers the edge, above the door, you might want to consider door skirts, like these. easy to make, and protect your hard work from extra wear.