The standard shower stall size is 3 feet by 3 feet. Most showers are larger than this to make them more appealing to home buyers, but regulations require the minimum be the 3 feet by 3 feet size so that people in wheelchairs can access them.
When someone designs a home there are regulations that the construction and sub-contractors have to follow set forth by the government. The average shower stall is usually larger than the 3 feet by 3 feet size that is the standard minimum. The average shower in most modern homes is 4 feet by 6 feet, which allows for additional storage inside the shower. An even larger shower may offer a no-door option, depending on the type of shower head used.