My doorless shower takes up about a 5' x 7' space.

Ours is 4'x7' with a 12" deep seat. Your size will also depend on where the shower head(s) are placed in relation to your shower entry. Will the shower head(s) be placed opposite your entry so that the water is spraying toward the door or will you place the shower head(s) on the side wall? How far down the side wall will they be placed?

Here's a pic of ours. We have had no problem with splash out at the entry or at the pony wall.

Hope this helps!