When you're looking into crewing on a racing sailboat, it's important to think through your options. Consider your experience level, physical ability and confidence.
Smaller boats are lighter and require less brawn, but often require agility and stamina.
Larger boats have greater forces and require more strength in some positions, but also have more space and important roles to be filled by lesser-experienced crew. Research what fleets are strong in your local area by talking to those involved in the sailing community. Post cards up on the local yacht club bulletin boards stating that you're interested in crewing on a boat. Discuss your experience level and goals with those who respond to your posting. Ask to go on a few practice sails, or to start crewing on a regatta of lesser importance to see if you fit in with the tone of the racing program and get along with the other crew members.