Rash guards come in short sleave and long sleave versions. Get one that is the right size for you. If it is too small you can still probably fit into it since usually stretch a lot, but it won't offer the same UV protection.
Short sleeve rashguards give you full freedom of movement. Not that your such a pansy you couldn't move just fine with a long sleeve. Still, it is slightly less constraining. Just remember to slap some bullfrog suntan lotion on those arms.