When you're trying them on, aim for a shoe that fits snugly enough that you don't get much, if any, movement in the heel, but isn't so tightthat the shoes will be uncomfortable when your feet swell in summer heat or when you're wearing thicker socks come winter. Bring the socks with you that you're likely to wear while cycling. For me, the deal breaker is always movement in the heel. I also went up a size in my SIDIS to allow for a bit more comfort come summer.