The International Residential Code stipulates a clear and unobstructed finished width of at least 22 inches for the shower opening. Be mindful that this is not the width of the door itself, but that of the opening it creates, and you may find a wider opening more comfortable to use. A swinging or pivot door typically provides a wider door opening than this minimum, while the opening provided by a sliding door typically starts at around 22 inches but can be bigger. Check the size of the opening before purchasing a door.