You have the definitive answer from the boss of Bee Thinking, but I have some additional thoughts.
If you look at any Langstroth 8-Frame box, you will notice that you could probably squeeze 9 frames into it. In fact, Michael Bush does exactly that, having first shaved a little off the wider part of the end bars of each frame (let’s call this part the shoulder). Most people don’t do what Michael does though, they put in 8 frames, “shoulder to shoulder”, and then position them in the middle of the hive. This means that there is a little gap between the shoulder of the edge frame and the wall, but it doesn’t cause problems - the bees just draw out the comb to create the correct bee space. If you have a box which is 1/4" bigger, that is only 1/8 inch each side of your grouping of frames - not enough to be an issue.
To give you another perspective, some people put only 9 frames into a 10 frame Langstroth super. I think @JeffH is one of these. The advantage of this is that the bees draw out the comb into longer cells, which means that there is more honey per frame (less work uncapping per weight of honey) and it is easier to uncap, because the cappings are not obstructed by the wood frame getting in the way of the knife.
So I agree that bee space is important above and below frames, and between the end bars and the wall. This is mainly to discourage bridging comb, which can make it hard to remove frames. However, when you have a little extra room next to the honeycomb face of the frame, the bees just adjust the cell length to make the space fit their needs.
In summary, I think your bees will do fine in the bigger box with the usual 8 frames.