Check for fit in the legs and feet and seat and cockpit. Some kayaks are designed for bigger people and some are designed for smaller folks. Sea kayaks should fit snugly, yet be comfortable, with the balls of your feet on the footpegs when you sit up straight. Go to dealers and test drive as many sea kayaks as you can. Or go on a tour and try a different kayak each day of the trip.
Some people rent a different boat every time they go out until they find the one they like best.
Tour operators and rental outfits often put used kayaks on sale and you might get a bargain. Join your local sea kayak club. The local dealer will probaby be able to tell you how to get in touch with the club. Once you re a member of the club you can check club sources for used kayaks. Attend a Paddlefest or Kayak Symposium where you can try out all sorts of kayaks and learn all the basics.