Unlike gorillas, orang utans and chimpanzees, gibbons are much smaller and both sexes are (of) the same size.
Is 'of' needed? Are there any errors in the sentence?
No "of" is not required. It's optional, but I would not use it.
I'm not sure the logic is entirely right with the "unlike." Are gibbons unlike the others because both sexes are the same size? It seems a bit off to say that one way they are "unlike" is that they are "smaller."
I prefer: "Gibbons are much smaller than gorillas, orangutans, and chimpanzees; and both sexes are the same size.