I'm just about old enough to remember (in the UK) foolscap paper, an imperial size also a bit longer than A4. You never see it any more (at least I don't) but foolscap sized box-files are still readily available. I guess a slightly bigger box than you need is not usually a problem.

Huh, this explains why some of my older teachers would call a4 pads as foolscap pads. I guess the paper size had been updated by the time I got to school, but their terminology wasn't.

Foolscap is just half an inch below legal - and I bet having a box a bit larger than what you're putting in is a selling point, not a hindrance.