are (of) the same size.

[Tan Elaine]

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?

Thanks.

 

[SoothingDave]

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.