Just as there's a big variety in the size of laps, there's a big range of sizes for lap quilts. They can be as small as 36 X42, and I have seen quilts referred to as lap quilts that I would have used on a twin bed. As far as your question on the number of blocks to cut, A 5" patch finishes at 4.5" and a 3" patch finishes at 2.5". You have made the quilt before, so take your original finished dimensions and multiply them by 0.5556 to get the new quilt size.