The sizes are based off arm length. If you order too short, on a tall person the blade looks like a butter knife. If you order too long, the blade is likely to pop your cap off. It is ideal to be measured in person, as the blade lengths can vary due to the arm length of the individual. However, if you are unable to make it into the shop to be sized for a sword, then it is best to size based off height. Below is a list we use for "general sizing".
5'6"-5'7"= 28", 5'8"-5'9"= 29", 5'10"-5'11" = 30", 6'-6'1"= 31", 6'2"-6'3"=32", 6'4"-6'5"= 33", 6'6" & over= 34"