Assuming tiles are 4" x 8", it looks like interior dimensions are about 32" wide and 24" deep. Consider the size of your dog when planning for yours.

I'm planning a dog wash. the space is about 36" wide (was supposed to be an alcove in a powder room for a vanity and sink) I turned it around to open to the laundry room. I'm thinking the drain should be 18" off the floor with another 9" 'front' to keep dogs & water in as much as possible. hand held shower on the right side (I'm right handed), & tiled up 44" on three sides. my dogs are 2 West Highland White Terriers; my vack's not getting any younger.