Jed, the size frame you need depends on your body type. At 5'11, probably a 19 or 21. I'm 5'8", but I have short legs, so I ride a 17.
The quick and dirty rule, especially for a newer cyclist, is to stand up with your feet on the ground straddling the frame. You want about an inch of clearance between the cross bar and your crotch. The reasons for this should be obvious. Road bike frames tend to be taller.
Other than that, while road bikes are better for long commutes, if you are planning to ride that thing on anything other than a well-paved asphalt, (ie sidewalks, brick-paved roads, etc.) get a mountain bike or hybrid. If you get a mountain bike, spring for slick tires. Don't try to scoot around town on a pair of knobbies. And always wear a helmet- mine has saved my shit more times than I can count.
Also, always buy a bike used.

