IMO unless you want a very small quilt, don't cut the layer cake. A 5" square would yield a 3.5" finished block IF 3/4" seams were used (large for ragging and shrinking.)
This has worked well for me - use 10" square, 3/4" seams AND sewing an X in the middle; sewing from one inside corner to corner.
Then wash a couple of times, stopping washer and dryer frequently during drain and spin cycle to clean out lint. Don't want stopped up drains or a fire in the dryer!