If you go to Western Bee Supply of Paulson, Mont., you can order 8 frame boxes in 4 sizes from 13 5/8 to 14 1/4. Since I make most of my own equipment I make mine at 13 3/4. That gives a little extra room for pulling the frames w/o rolling the bees. With 1 1/4 frames you can fit 9 frames into the 14 1/4 and maybe even into the 14 0 size.
Standard dimensions were 13 3/4, but back in the 50's and 60's 8 frames became somewhat regionalized as 10 frames became the dominate standard and 12 frame and 8 frame lost favor, so the dimensions varied depending upon which regional manufacturer 8 frames were purchased from. 12 frames are still out in the cold due to excessive weight when full.
Users of 8 frames became to be viewed as "lesser" beekeepers being ignorant hold overs or crackpots. It has only been in since the mid 90's that 8 frames have begun to re-establish themselves as a desirable option. Up until 5 years ago some of the major suppliers of bee equipment still didn't offer 8 frame equipment, but now I think all the major manufactureres do.
I have kept bees in 5 (nuc), 8, 10, & 12 frames hives. I have also used Dadant Deeps (aka double deeps), Deeps, & Medium boxes.
My final selection of which equipment I used was predicated upon how well bees preformed year around in which size hive.
12 frames were out, not only due to excessive weight but because the bees had to be forced (frame manipulation) to draw out the outer 2 frames in the 12 frame boxes. Over winter losses were very high due to, IMO, excessive space.
10 frames worked well but could still be a weight factor when using deeps. The bees drew out all the frames, although the outer 2 frames lagged somewhat, and were often abandoned when supered. Winter losses were about 1/2 that of 12 frames.
Bees in 8 Frame hives seemed to build up faster, required more frequent supering, had larger brood chambers (8 frames verses 6-7 frames in a 10 frame), overwinter with little loss, and were the easiest to manipulate as the center of gravity was closer to the body (being narrower) and slightly less weight. It is not uncommon to find 3-4 boxes, 8 frames each, of brood in my hives mid-flowduring the 2 heaviest flows in my area; raspberries, strawberries, blueberries in the spring and blackberries mid summer. I found that in production I could harvest 2-3 extra supers off of an 8 frame hive than a 10 frame hive, given good forage grounds. That is a harvest increase even with the difference in frame count per box.
I went to mediums after receiving serious back injuries while I was a Police Officer which forced my retirement from that carreer because weight became and issue and the narrower box with it's center of gravity closer to the body an added plus.
Here's one for the Commercial Beekeepers to ponder. Given the dimensions of 8 frame hives, the pallets required are smaller than with 10 frame hives. Using the dimensions of a fairly standard 18 wheeler flatbed, it is possible to put one etra pallet on the truck for the length of the bed. If you're placing 3 pallets wide, then the extra amount of hives per load depends on whether you're using single or double boxed hives. An extra 9-12 hives peroad is possible, add a trailer with the same dimensions and you've increased your single trip haul by 18-24 hives lowering the transportation cost per hive. The profit margin increases because the ratio of pollen fees to transportation costs also increases--less cost per hive, more pollenation fees per trip and you still get the 8 frames of bees per box rate.