With the stated dimensions you could easily frame a pair of standard 9x7 doors in the front, including full, double 2x12 headers, into the existing opening. The issue is that the ascetics would be pretty awful, with two large doors nearly touching. The idea to go with something like a 16x8 is probably best in this situation, kendallf is correct, it involves a pretty substantial engineered lumber beam to replace the current set-up.
If I was tackling this, I would oversize the beam as much as practical, which really isn't a big deal, since upsizing a beam isn't too terribly expensive. If you go to an actual lumberyard with the dimensions, and other info (roof pitch, building depth, wall thickness, no second floor or attic loads, etc...) they will provide free engineering from their supplier. The other thing I would recommend is to go with a professionally installed door with the commercial torsion style springs. It's a cheap upgrade, and eliminates the more traditional extension style springs, which suck in so many ways. At the start of my building career I lifted the garage door of my client's nearly finished house, and heard a noise like a bomb going off. The extension spring broke, kinked a substantial metal bracket, and shot into the drywall, like an arrow. That was the day I switched to torsion springs.
