Standard PCB dimensions

Predefined standard in PCB sizes is simplifying assembling a lot. You will spend much less time to make housing and connect blocks between each other. I used 25 mm as module for years, but you can use 25.4 mm according to industrial standards, since metric system for most of electronic parts is deprecated. Side proportions can be 1:1, 2:1, 3:2, 4:5, i. e. multiplies of module.

If smaller side is kept same on all modules in device, it makes connections between them easier. Standard dimensions are also mean standard mounting holes placement. Anyway, if your device is mounted on single PCB, you can pay no attention to standard to keep dimensions small.