Given you are going to make the frames from 1x stock, you'll need to make the door opening at least 34". The height of the door opening should be 81 1/2" off the floor. You'd put the shoulder on the bottom plate, and the cripples/jacks/shoulders would be 80", then you'd head those off.
Remember one thing, though, when in doubt, you can always make your openings bigger and use a filler to make them smaller. If you make them too small, there is no return.