What type returns on the jambs are you going to use? Wood, sheetrock?
Reason I'm asking is your suggesting just using a piece the same width as the opening. Not a great look!
If your adding wooden jambs and casing around the inside of the opening the top needs to be long enough to extend out past the outside edges of the casing. Go look at the window stool on one of the windows in your home to see what I'm talking about.
If your doing sheetrock returns, same thing you want it to run past the inside corner at the bottom by at least 1/2.