La Sportiva's Guide to Fitting Rock Shoes
While it isn’t a black or white issue, a general rule dictates that soft shoes require a tighter fit, two to three sizes smaller than your street shoe size. We recommend you fit your trad shoes one to two sizes under your street shoe size. A thick midsole creates a platform on which you stand, while a slip-lasted slipper-type shoe relies on your crammed toes to create an edging platform.
Once you determine your needs, try on several boots in that genre. Different last types fit different feet -- that's why we make lots of lasts. If it feels good, it probably is. Remember to factor in stretch. Unlined slip-lasted shoes tend to stretch a bit more than lined shoes -- about one full size. Lined shoes stretch about a half size. Remember: a well-fit comfortable shoe will work better than an ill-fitting tighter shoe.