I have found for rag quilts 6 1/2 inch blocks work best because you need a larger seam allowance. If you are making traditional or non-rag blocks 5 inch works well.
If you can get 2 of each block you can make an "I-spy a match"
quilt to add another game. You can also make "themed" quilts for holidays, summer, games, sports, foods, etc. These take a bit more time to collect, but are fun.