I do 1/2" on smaller blocks and 5/8" on larger blocks. You can use any size you like just like any shape or size squares you use. You can use the denim alone or with a layer of flannel (saw the one layer denim on Etsy I think they called it a picnic blanket) and the denim will get heavy very quickly. I found a great tip here (can't remember from whom) to make it in sections and snip the seams as you go. Don't snip the seams you are going to be sewing other sections too. An example would be, if you have 20 rows, sew 4 sets of 5 rows and then sew the 4 sets together. Unless it is a very small quilt take it to the laundry mat for the first washing because you could stop up your washer. The raq quilt snips are a must. I saw a video on You Tube for a denim rag quilt made from jeans using a rectangler shape for the blocks.