Are Jewish men allowed to show their arms?
Yes off course only women
For women the sleeve must go past the elbow (according to the most lenient opinions, a fist's-length above the elbow - but that really shouldn't be followed). For men, tzniut is less defined. I generally like to wear collared, long sleeve shirt (but occasionally I will wear a t-shirt). For men, the way one should dress is more about the fact that one is always standing before a King. (Lubab, correct me if I'm wrong with any of this)