The concept sounds quite similar to a hugely successful Boston mini-chain, Anna’s Taquerias. Was once written up in the NY Times online as worth a day trip to Boston and even has a Wikipedia entry: http://en.wikipedia.org/wiki/Anna's_Taqueria
Sort of an abbreviated version of a Mission Taqueria. Counter at the first one is maybe 12 feet, including register. Two types of beans, rice, maybe 5-6 fillings (at least one being a good veggie option), couple of salsas. Anything can be served burritoed, tacoed, etc. But it’s all their own and fresh. Qdoba tastes like Old El Paso cans by comparison. A friend from LA was complaining about there being no good burritos in Boston and when I mentioned Anna’s she said, they’re good, they’re just too clean.
I recall reading an article when they were first taking off that the owner was shortly out of college, from the Bay Area, and had a relative with a successful burrito chain. Went looking for an area with a young receptive population and picked Boston (actually started in Brookline). Picked the name Anna so it would be near the top of phone/restaurant listings. First location was probably almost the size you’re talking about, then picked up the lease on the store next door for more dining area and then opened another a half-mile away to pick up the overflow and expanded foot traffic. Many others have sprung up since then, Herrera’s even serves out of pushcarts in the business/shopping district, but Anna’s seems to maintain its niche with reliable quality, cheap prices, and targeting areas with college students.
Of course, that’s Boston, where it’s not that long ago that Jack in the Box was considered Mexican. But the model might be worth a look.