I bought a polycarb box to house a PCB I'm creating. IP65, marvellous. In in the corners inside the box are threaded stand-offs, but no documentation was supplied and running the product code through google hasn't helped. Is there some sort of standard defining case sizes and the centre to centre dimensions of the stand-offs?
I'm afraid you're on your own. A single manufacturer may have a hundred types of enclosures, all with different mounting dimensions. If you can't get a drawing(*) use you calipers to measure the distance. First the minimum distance measured inside the holes, with the outside jaws (1), then the maximum distance with the inside jaws (2). The average of both readings is the center to center distance.
(*) I would start with downloading a mechanical drawing of the enclosure before I bought it. No drawing, no sale.
No spec no sale doesn't always work, sometimes you don't get to be picky.
A way around not being able to use calipers on holes recessed in the case is to buy some long screws and put them in the holes. This depends on the threads being very close to normal to the back plane of the case but I found that this assumption was valid for the cases I needed to measure.
When manufacturing several identical units it may be worth buying an extra case and butchering it until you can reach with the calipers, and also keeping your own datasheet. I have taken to printing a label with all this information plus the supplier and product code and sticking this inside the lid.