A walk in shower cubicle is recommended to span at least 36 inches by 36 inches, including a bench that is fixed to a wall.
Such a size actually offers a very good basis to work from when installing a walk in shower cubicle, whether or not a shower bench is a feature of the cubicle or not.
For more insight and inspiration into walk in shower cubicles, take a look at our related blogs – 12 Wonderful Walk In Shower Ideas, and our Shower Cubicle Buying Guide.