"Odd" sizes used to be pretty uncommon, but these days there's odd-sized panels from numerous companies. In fact, some have whole lines of odd sizes, such as Erica's Pico line (all 3 hp). Konstant Labs even makes an actual 1 hp module, their PWR Checker, which is super-useful for those who want (but don't have) rail status indicator lights.
Elby also does quite a few odd-sized panels in their Eurorack and Euroserge lines; the idea with these is probably that if you're planning to use Elby modules, you're apt to use several, and the "oddness" will get resolved in the build.