Trail running shoes should fit snugly, but not too tight, in order to provide the necessary support and comfort while navigating uneven terrain. Your toes should have enough room to move and wiggle, but not so much that they slide around inside the shoe. The shoe should also hold your heel securely without slipping, to prevent blisters and other injuries. It’s best to try on trail running shoes towards the end of the day when your feet are at their largest, and wear the socks you would use for running when trying on shoes.