I'm just about finished quilting a double that started out as a lap size. Oops. :) I had intended to put borders on it, but decided against it. It's already WAY bigger than I planned and borders would just make it worse!
I *have* to finish what I start - it's a compulsion. I have a supply of batting (random kinds) that is almost as big as my supply of fabric (but seems bigger because batting folds bigger than fabric...lol). I will sometimes do a few small projects in the midst of a big one, but I always finish those too, before moving on.
For quilts (lap sized +), I use bed sheets for backing because I hate piecing the back - they're always big enough! :lol: I just take my quilt top (or swatches of the fabrics used in it) shopping with me and find one that coordinates. I also will pick up flat sheets on sale when I see them, or even sheet sets, and cut the elastic corners out of the fitted sheet. I try to stick to neutral colours or colours I *know* I have in my stash so I know it will match *something* and get used. lol