look for something with a decently sized tweeter, at least an inch but not more than two and a half inches and look for a driver that's about 5 inches and no more than 6 1/2 inches. If you get too big a driver, you miss alot of midrange.
decent models include the Infinity Outrigger and Outrigger Jr and Polk Audio Atrium 140. you also need to look at the weight for what you have to support. some speakers weigh 15 to 20 pounds each...
one last thing.. Yamaha makes some really good outdoor speakers, but they are 6 ohm rather than the normal 8 ohm, so you need to check if your receiver will drive them.