Do whatever makes YOU happy butterfly /tribal full sleeve it your body your art enjoy it don't worry what others say. Or, just get muscles tattooed.lol
It is mostly depend upon your thinking but it is very important to look out the tattoo artist which you choose.
placement would be very important coz i think tatts can sometimes make skinny arms look long as shit!! i wouldnt get it on your arms if they really are very skinny, just hit the weights man itll be good preparation for the pain anyway so you can bulk your arms up while you think about what and where