Once you’ve decided on the kind of shower you want, narrowing down a location is easier. For simple rinsing stations, the requirements are minimal: access to a water line and basic drainage. But for a full shower setup, you’ll need to find a place that has good drainage (often a drain will need to be installed) and hot and cold water lines.
If you have a bathroom that shares a shower wall with the outdoors, use that for your outdoor shower. It’ll be much easier for your plumber to connect the water lines. Do you need a shower enclosure? If so, you’ll need to factor that into the space as well.
Make sure your drainage system is set up properly. If a basic outdoor shower is being built over sandy soil, you may need only an opening in the flooring. But any soil that drains slowly will require a more elaborate system and usually the help of a plumber.