There is no definitive answer to this question as the ideal opening for a walk in shower will vary depending on the individual's needs and preferences. However, a good rule of thumb is to make sure that the opening is at least 32 inches wide so that there is plenty of room for maneuverability. Additionally, it is important to ensure that the opening is tall enough so that users can comfortably stand up straight without having to duck their head under the showerhead.