A shower pan serves as the foundation of the shower -- keeping the water away from your floorboards. Making the wrong decision here could lead to problems in the future with leaking, damp, and rot.
If you are replacing your current shower tray, then the easiest thing to do is measure the dimensions of this one before disposing of it. For a square or rectangular pan, this is quite straightforward, for a quadrant base (neo-angle and semi-round), you will also need to know the radius measurement.
If not, then measure the inside length and width dimensions of the shower floor and then the same from the wall studs to the drain hole. Don't forget to take into account the position of the drain when selecting a suitable pan.
You are also going to want to think about your preferred threshold for the shower base or pan.
Barrier-free shower pans allow you to step right in, without the risk of trips or falls.
Beveled thresholds are slightly sloped down allowing for wheelchair access.
Curbed thresholds are the more traditional kinds. They prevent water from leaking out of the shower base. These are available from ½ inch to 6 inches in height.
Temporary thresholds enable you to either have or remove the threshold depending on the needs of the person using the shower.