Cable Tray Sizing

Not certain that there are any such regulations or guidelines.

Cable tray must be able to support the weight of the cables, with a margin of ignorance. The limiting factor is often the fixings rather than the tray itself.

In most circumstances I would suggest that tray should be wide enough to accommodate the cables spaced apart by one diameter as this will reduce cable derating for grouping.

If however the cables are very oversized for voltage drop or other reasons, then spacing becomes un important. 

It would be good practice to allow space for additional cables to be added if needs change, but that is merely good practice and not a regulatory requirement.

In many cases, the customer, architect, or consultant will have requirements as to cable tray size and design.