Another thing to add. It works great for use outside of the tub as well! I also use ours for a quick seat when I have to go to the bathroom – sometimes you just don't have a free set of hands!
It's about 1ft wide and 1.5 ft in length. I ended up buying 3 different ones (we found this one is/was the best quality but most awkward to use from newborn till about 6 months)