Do enclosed switching power supplies follow any standard for their chassis size and mounting holes distances?
or each manufacturer has its own conventions?
Not generally no, they don't have the same sizes or mounting patterns. On occasion some companies will match the mounting pattern and size to compete directly with a power supply. An exception to this is industry standard supplies like ATX or supplies that are built for 1U chassis ect.
It's a good idea to put a power supply on a bracket that attaches to a chassis in the event the power supply needs to be switched in a product. (I've had a few power supplies blow up and we had to find a replacement, and because we had a bracket we could make them field serviceable)