It's plenty big. Consider that a fairly common shower base size is 48"x48" which has virtually identical square footage to what your architect is proposing.

Best advice ever - "BTW your architect is not a bathroom designer or kitchen designer so get those pros involved now".

That is big enough, but I don't think it is big enough with a seat. A seat will take up a minimum of 12" of depth. Unless maybe a small corner ledge would work? To a certain extent this is personal preference, so block it out and try to get a feel for how you will like it.

Yes and yes. :-)

Thanks all for input. I'm trying to think out of the box (or in the closet box) for a crazy idea that may be a bad architectural or design idea. I'm adding two walk-in closets in our design. One is behind the end of the shower. I was wondering if it made sense to give the back 24" of the 2nd walk-in closet to the shower for a 24" wide (16" deep) seat. The remaining 18" width of the 42" shower backs to a small linen closet which is only 12" deep, and I don't want to make that any smaller). So I would have a 24" wide by 16" deep seat nook across part of the end of the 42" wide shower.

If you go with a portable seat it is large enough. You can get a shower seat with adjustable leg height and suction cups to make sure it doesn't tip. It can be moved out of the way when not in use. I've seen 48"x48" showers that can fit a shower seat and 60x30 ones that can fit a shower seat. So, 55x42 is enough width and length.

Best way to really visualize is to take some boxes and masking tape and build the size of it and play around with chairs or something to see how much space you have.

Yeah, I did the box things when we opened up a wall and had a "demising" wall cabinets installed. I wanted my wife to see how it would look, and the box thing worked.