If you are lucky, you can consult an experienced bootfitter. A good one will measure both feet, both seated and standing with a Brannock Device. This allows the bootfitter to determine your natural foot shape versus your weighted foot shape. It will very quickly show if there are expansion issues due to a fallen arch, either in terms of length OR of width. Both issues are common. Also a truly experienced bootfitter will be able to determine fit issues that can't be measured by the Brannock, such as width through the instep and heel and the volume through the bridge of the foot.
Also, a good boot fitter will know the general fit characteristics of the boots in question, particularly which ones are cut wider/narrower in the heel/instep, again areas that DON'T get measured by a Brannock.
But all this is secondary to trying the footwear on an incline. One critical area nobody has mentioned is walk with the shoes on an incline. This really is a critical fit issue unless unless you only intend to hike on flat pavement.
Walking uphill exxagerates your heel movement, but you should have no more than maybe a quarter inch (5-6 mm). You can reduce this movement with a plethora of lacing techniques.
On the downhill, (which is the more critical of the two checks IMO) you should be able to walk in place without your feet sliding into the front of the shoe. A shoe that is too wide will allow such slide even if you have the right length. But if you enjoy keeping your toenails, avoid this in your boots.
If you had issues with substantial foot expansion due to a falling arch, an orthotic such as Superfeet may be helpful, and may prevent you from "toe-touching" on a downhill. However, getting a pair of shoes that truly fit your feet, both for length as well as width and general footshape is a much better solution than the host of inserts, sock thicknesses, and lacing techniques to work around a not-so-great fit.