I think of the width of the rail and stile as 2 inches, plus the width of the "moulding" which on all my cutters is 3/8 inch. Then I make the doors 1/8 inch oversize so I can trim for absolute square, and to get a pretty joint between the rail and stile (no glue in the end-grain of the stile). So my rough lumber is cut to 2 1/2 inches.
If my tenon and groove are 3/8 inch deep (standard among my cheap cutters), then I simply assume the rail is 2+2 inches shorter than the door width. But my "good" cutters have a 1/2 inch tenon and groove for extra strength, so my rails have to be 1/4 inch longer.
The 2 plus 3/8 inch gives a good amount of room for the cup of a Euro hinge, plus an edge profile on the door. I sometimes cheat it down a little - perhaps by a 1/4 inch, or more if there are no hinges. Wider starts to get into issues of wood expansion - cross grain on the rails glued to the long grain on the stiles, so wider may be weaker, not stronger.
I have made a spreadsheet for doing all the door calculations. The instructions and the link to download the sheet are at
www.solowoodworker.com/wood/doors.html. Somebody on the web was selling a spreadsheet to do that for $25 - mine is free.
My spreadsheet also does the calculation for floating panels, with and without space balls, compressed to the factory recommended amount, etc.etc. Same for glass and flat (plywood) panels.