Free yourself to wear short sleeves without worrying about your arm skin.  Solbari, an Australian company, has developed arm sleeves made from a cotton, bamboo viscose and elastane fabric blend.  Said to offer the same protection as SPF 50, they are lightweight and comfortable.  So throw on that T-shirt or short sleeve polo, pull on your arm sleeves and know you are being sun smart - whether you are riding, playing tennis, golfing, boating, cycling or even gardening.  #FreetheSleeve.  

Model is  5'10" / 120 lbs and wearing a pair in size Small - which fit her well.  (Her review was that the sleeves are very comfortable.)  Machine wash cold, line dry. Do not tumble dry.