Many folks use 2 1/2" strips because of precuts and because it is an easy size to use. BUT - you can use whatever size you want. I have seen some great looking quilts that have giant logs and some made up of very tiny logs. What are you most comfortable with? If you like to do a quilting scroll in each log, then perhaps a 3" or 3 1/2" cut strip would be better. What is your stash full of? If smaller bits, then use a narrower strip. If you use the 'darker' side as a 2 1/2" and the lighter side as 1 1/2" strips, you will get the curved log cabins. Always remember - IT IS YOUR QUILT and YOU get to make the decisions.