It is recommended that a walk in shower spans no less than 36 inches by 36 inches, this inclusive of a bench attached to one of the walls.
Regardless of a shower seat or bench featuring, 36 inches by 36 inches represents a good minimum size foundation to work from to allow for comfortable space for easy showering.
Take a look at our blog concerning 12 Wonderful Walk In Shower Ideas for a more detailed insight into walk-in shower enclosure features and profiles.