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Are you looking for door sizes or for the actual openings in walls etc?
Is it for an internal door or an exterior door?
Is the door going to be fire rated? If so to what standard?
Finally if I'm honest I couldn't tell you if the is a BSI document about door sizes. I'm sure there is a minimum size of door openings for a number of different circumstances but I am never involved in specifying this so couldn't tell off the top of my head.
Doors do tend to come in standard sizes, these are normally metricised measurements taken from imperial, so it normally makes more sense to work on doors in imperial.
I have made doors and adapted them as well as door liners or frames to fit so many different apertures over the years in both new build and old stock housing that I can say that it's very easy to justify moving away from any standard sizes that exists as long as other building regs are met (fire doors etc).
Might be being a bit thick, but can't find the sizings for door openings/apertures. Sure as hell ain't paying to read the BSI.