These all seem to be perfectly fine. In English, sentences can be constructed to have the same meaning using different wording, with perhaps slight differences.
"a" suggests that there is a left arm and a right arm, and that both are of equal importance.
"b" and "c" suggest that there are left arms and right arms *plural, though "a" can also mean "all left and right arms" in the proper context), and that they are of equal importance.
"d", "e", and "f" suggest the same thing as "a", "b", and "c" respectively, but the word "similar" instead implies a sameness and equivalence in the arms, but not an equality or even any importance (greater, lesser, or any at all).
"g", "h" and "i" likewise are similar to "d", "e" and "f" or "a", "b" and "c" respetively, but instead of suggesting a quality, informs a specific fact about the arms (first the size of the left arm (or arms), then the size of the right arm (or arms)).
If you only want to speak about one set of arms, you would use a, d, or g. If you wish to speak about several sets of arms, you would use b, c, e, f, h or i depending on what you want to say about them.
If you want to talk about all arms in general, a, d, or g would likely be preferrable. However, in case g, h, and i, one will likely infer that you are talking about two specific arms, because arms tend to come in a wide variety of sizes. Stating two specific sizes lends one to believe you are talking about a singular set with a specific size.
None of these 'answers' are the 'correct' one, they are simply different ways to construct sentences with slight variations on what each one means.
