길고 짧은 것은 대어 보아야 안다
You have to measure it to know if it is too long or too short.
Used to encourage someone who is unsure if he or she has the ability or skills needed to do a certain task or meet a certain challenge to accept the task or challenge to find out for sure.
잘하고 못하는 것은 실제로 겨루어 보아야 안다.
You will never know until you try.