Usually this size is based on the size of common bathtub, because the function of this curtain rod is not only for shower but it can also be used for bathtub. There are several things to consider when you want to choose a curtain rod for your bathroom. You should measure the length and width of the spot that you want to be curtained. This is necessary because you the size of there are different size of curtain rod; there are ones that are 66” long and there are also ones that are more than 66” long. Bathroom.