Door frames & doors

I am perplexed by doors & door frame sizes. I need to install 3 internal doors with new frames. 2 of the doors/frames will be in new walls (which aren`t built yet) so can be any size I want. The 3rd door will need to go in an existing opening 6ft 7in high by 2ft 11in wide. Is there such thing as a standard size for internal doors (new)? Obviously the existing opening will govern the size of the 3rd door/frame. With this one, do I just buy a standard frame/door from the sheds & cut down to size? I would like plain solid doors (no panelling) that will be painted & won`t warp !!
TIA Steve
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