What I usually do when making a lap quilt is figure out HOW the person sits watching tv or reading a book. If they are stretched out on say like a recliner, you want it a bit long to cover their legs, so something maybe 45x50 would be a good size. If they curl up, then maybe 45x45 would be good. If you have tile in your house, a way to "measure" so you don't have to take out your measuring stick...most squares are 12x12, so you can pretty much figure it out that way. I count the tiles on my floor ALL the time when figuring out how big I want to make a quilt.
Patterns can be found all over the internet. Just search for Free Quilt Patterns on Google.