Need a bit more info to answer...how big was the one you have made several times with 5" blocks? How many 5" blocks wide and long? If you have that measurement you will be able to predict how big the one you make with 3" blocks will be. There is a range of sizes that fit in the lap quilt category. I make them anywhere from 40" x 50" to 60" x 70". Sort of depends what the eventual use will be. A "lap quilt" for a large man who will use it while he watches TV in his recliner would be at the high end of that range. A lap quilt used by a smaller person (or a child) would be at the lower end.