Installing the shower pan and any doors included yourself is. Typically the smallest size of a prefabricated shower unit is 32 inches x 32 inches even though you might find some in smaller sizes. In general a shower should measures at least 36 inches square. You can purchase shower kits in various sizes ranging from 36 to 42 inches square. This means we might not get to see the same standardized size of shower head threads all over the world. To determine the minimum shower size youll need step inside a few showers at a plumbing showroom or home improvement store. Shower and Bathroom Size The other aspect you need to consider when choosing your shower size is the size of your bathroom as a whole. The size and location of the fixtures in your bathroom can. The latter size is the absolute m inimum size for a shower. Standard Size in the United States of America USA In the United States the optimal size of the thread is ½ half inch 127 millimeters.