Shower stalls in standard cabins are not especially large - maybe 30"x 30" or so. It isn't until you get into the suite categories that you get a bathtub/shower combo which offers lots of room.
As for chairs...you will find some with arms and some without, and there are lots of bench-type seats to be had.
I have seen many people on cruise ships who, to put it delicately, were excessive in their girth. They didn't seem to have a problem finding seating which accomodated them.