Feet swell over a long day, so try boots on in the afternoon and wear the socks you will hike in.
The thumb test
Unlaced, slide your foot forward until your toes touch. You should fit one finger behind your heel. Laced, your heel should not lift more than a few millimetres on stairs.
Test downhill
Toes hitting the front on a descent means the boot is too short or the lacing too loose across the instep. We have a ramp in the shop for exactly this — see the size guide or drop in for a free fitting.