Check out 1000bulbs.com and look under standard CFL bulbs that will give you an idea of what's out there. If you only have room for two bulbs, I would definitely go for something bigger than 26 watt or you will be greatly disappointed with your final yield.

With only 2' vertical space, I would probably mount the sockets on opposite sides of the grow space rather than the top, one toward the front and one toward the rear of the cabinet, with the bulbs horizontal - that will maximize your available height.