'~ 정도면 (이면) ~ 이다 (하다)' is a rhetorical device for emphasis. For example:
네가 미스 코리아 대회 나갈 정도면 난 미스 유니버스다. If you are pretty enough to participate in Miss Korea Beauty Pageant, I am Miss Universe.
This sentence jokingly means "Sorry, you are not that pretty." or "I am far more beautiful than you think you are."
'정도' means 'degree' or 'extent'. Your example sentence is literally translated to:
(너의 친구들 키가 커서) 네가 작아보일 정도면 친구들이 걸리버야 If you look short (because your friends are extremely tall), your friends are Gulliver.
This sentence means
You are not that short. Your friends are extraordinarily tall. That's
why they make you look short in the picture.
The sentence could be rephrased without using the noun 정도.
네가 작아보인다면 친구들이 (친구들은) 걸리버야.
Another similar expression:
네가 국회의원이면 나는 대통령이다. If you are a member of the National Assembly, I am the President.
This sentence means "I know you are bluffing about your position". "Don't give me that bull shit. I know you are not what you say you are."
You can make similar sentences using this construction.
네가 경찰이면 나는 국회의원이다. If you are a policeman, I am a member of the
네가 부자면 나는 빌 게이츠다. If you are rich, I am a Bill Gates.