IMO, the smallest usable dimensions for a pool bath/changing room with a sink is either 5x5 or 6x4. I've done a ton of them in structures or tucked into a nook of the garage.
These sizes are very cramped and offer no cabinet space.
Your structure's size will be pressed to incorporate a boxed corner of either size. Jumping up to 22x18 doesn't gobble up much more lumber or shingles and will take a 6x4 without giving up much feeling of spaciousness. If your structure's roof is hipped it'll give you a more defined ridge too.
I'm not trying to imply that your current size will be small but I'm really curious, what size of a bath/changing room does your builder propose?

See ya,
Kelly

More background information...we started with the 18x 20, but then decided it had to have a small bath. I believe the bath right now is 4ft 9inches x 6ft. Since we now have the bath taking up some of the space, I keep thinking it needs to be expanded. I have mentioned my concern and they said it could be larger. I didn't ask about how much cost there is to do the increase....guess I should. The roof is hipped....The ceiling inside will pop up in a section(10ft to 11ft)
Would you go 20 x 20 or 18 x 22 to give more room? I also had been thinking about recessing some space in the walls to give added shelving in the bath. Do you think that would help?

I would discourage you from a 20x20. IMO, they're painfully square. Although you'll gain additional space from the size I think your better off with a longer rectangle than a larger square.
Another very good size for yours would be 16x24. Most improvement contractors will charge you by the sq/ft. I look at both sq/ft and material consumption. On a jump from 20 to 22 long even though the square footage jumped to 396 from 360 you'll use the same piece count on decking, maybe a bundle and a half more shingles, and about a half yard more of redimix for the floor.
I'll see if I can find you some pic's of the look I like, maybe your man will have some similar inventory. I like a completely cathedraled ceiling with exposed rafters and exterior siding flipped over for the roof decking, it looks great and cuts incurred construction costs by not having soffit.

See ya,
Kelly

Huskyrider...Thanks for the input. So you lean towards a 18x22 or 16x24? I was going for a soffit for lighting and pop...but I understand what you are saying. I would love to see any pics you might have if it's not too much trouble.

What type of upper cabinets have you used?