Is it correct that to visit a Cambodian temple we need to wear a long skirt and a long sleeve shirt?

The basic rule is to cover your shoulders and knees (both sexes), and ideally you'd cover your ankles too. You are suggested to wear longer shorts, capris, pants/trousers or a knee-length-or-longer skirt on the bottom, and choose any shirt that fully covers your shoulders on top. A T-shirt, blouse or polo is fine, while a tank top is not. Some temples will not allow you to wear socks so sandals are the best.