It should be 2″ bigger than the door size. Even though it’s a rough opening it should be reasonably plumb and square.
If the opening was framed by someone else, you may want to break out your level and framing square and check this also. Drywallers sometimes believe the rough opening was meant for them and will let the drywall run into the opening. If this is the case use a drywall saw or sawzall to cut it back.
Once all the vitals have been checked your ready to hang a door. Prehung doors come assembled a couple different ways. They can be bought with trim already mitered and nailed on to one side and without trim.
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