I would think about putting in a large curb-less shower that will allow for aging in place. If you have back problems now and can't use a tub what happens if your back gets suddenly worse for even a short time and you need assistance in the shower? I can tell you from experience it can happen suddenly and in your thirties you don't need to be old.

I think I'd also do the 6' shower--you can then put in a bench should you need to shower while sitting. The hard part of cleaning is in your choice of materials more than your overall size.

Also, 4' by what dimension? I hope at least 4'x3'. I really like the size of 5x3 Kohler cast iron pans, if that is an option if you don't want to go the full 6'.

I like 3x5 too for it gives enough room to fit a free standing teak bench in the future if needed. I get cold in big open showers very easily, not sure if it is in my mind or not but I always feel drafty in one so I would prefer four feet to 6 plus feet unenclosed.

I have a 4x3 shower in my current bathroom. It's fine but nothing special.

I've been measuring showers in hotels and friends' houses for several years, and I really think 5x4 is the perfect size. Big enough for a stool ... or even a hospital chair, if necessary. Not so big that it's cold.

The issue that hasn't been addressed: Is a smaller shower and no tub in keeping with the rest of your house?

@aok27502 Would you mind sharing pictures of 4x3 shower in your master? I'm trying to do a small bath addition with just shower, no tub.

I have a 3x5' kohler shower pan in my bath. I could imagine it being 4' long though. In my 5' set up I do have a portable stool that is aluminum and plastic seat that I love. It might be all right in a 4' shower. The other positive with a 5' shower space is that in the future a tub could easily fit in that space. But as far as having a 4' shower, I think it would be very serviceable. Like you said, easier to keep clean and dry. Here is a link with a few pics of @cat-mom's 4' kohler pan. It looks very handy.

ETA, I forgot the link :( so here it is. Cat mom's shower is posted in this thread:

http://ths.gardenweb.com/discussions/2302277/for-those-of-you-with-the-kohler-cast-iron-shower-pans?n=14

Enduring, you make a very good point about a tub fitting into the space of a 5' shower space. Something to think about.

Rhonda, I forgot the link so I edited my post with @cat mom's shower pictures.

Our new master bathroom shower is 3.5' x 5.5' (internal - actually usable) dimensions. I think it could definitely be 3'x4' and I'd use it 99% of the time the same way, but I never sit on the (movable) seat. I'd definitely want 5' or preferably 6' length if: (a) 2 people might use the shower together (definitely put a shower head on each end in that case - slide bar for one so it can be low for use when sitting); (b) showering the dog; (c) aging in place (you'll need to use the seat, and will love having a handheld near it, lower; also someone can much more easily help you bathe; walker space). For aging in place I'd do a removable seat, not built in bench. Far easier to help someone bathe if the seat is in the middle of the shower, not at once end, and some persons do better with seats that have handles to help them get up/down (I think its too much to contemplate to build permanently if you aren't in need yet).

Re Width. We have 3 built in niches (and no stuff anywhere else) so 3' width would be fine. If we had shelves sticking into the usable width, I'd want 3.5'.