What size is LT in clothing?

I believe that would be size Large/Tall.

Generally it is 42 Long. 42 being the circumference of the chest. Long being that the person is more than 6'2". That is typical but different brands make different cuts.

A person wearing LT would be built like me - lean to mid-sized and tall. I am 6'3" and 200 pounds and never really was an athlete or a big gym guy - just a regular build is the best way I can describe it.