This article is from the Hiking Boots FAQ, by Chris Gilbert email@example.com with numerous contributions by others.
Most people will require a boot that is larger than their normal
shoesize. A good rule of thumb is, while wearing a pair of socks
that you intend wearing with your boots, put your intended
purchase on your foot *without* lacing it up. Now push your foot
forward so that your toes touch the toe of the boot. If you can
comfortably fit a finger or thumb into the gap between your heel
and the heel of the boot then the boot is probably the right size.
The toe gap allows you to descend a slope while taking your body
weight on the instep of your foot rather than your toes. Move your
heel well into the heel cup of the boot and lace it up, making
sure that you're not lacing too tight. Walk around the shop to see
if the heel of your foot 'rises' within the boot despite being
laced up. A significantly rising heel will probably blister on even
the smallest walk and the boots are unlikely to ever be comfortable.
The rising is caused through a combination of the stiffening of the
sole of the boot and too large an instep gap in the boot cavity.
The foot flexes away from the stiffened sole into the instep gap
causing the heel to rise. If you have already bought a boot that
permits too much movement consider fitting it with a padded footbed
to reduce the amount of space in the boot. Extra socks may also help
but the footbeds will reduce the amount of vertical space in the
boot without affecting the other dimensions. If a footbed
uncomfortably restricts the space available in the toe of the boot
then consider using heel pads. Some insoles double as a shock
absorbing medium. Specific brands include Sorbothane, Eagle Rock
and Superfeet. The author's own preference is for Sorbothane which
has been found to be both extremely comfortable and very long