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