The minimum I've seen is 5 foot wide... Depending on where you are, you need a minimum of 24" in front of the front of the toilet. So, toilet placement is important (as well as if it is a round or elongated bowl). The 8' dimension is not a problem. A standard small bath with sink, toilet, tub/shower is 5x7.

You need a 30" minimum width for a toilet plus the front space as Kirkhall mentioned. The other 18" width could be used for a sink mounted on the same wall as the toilet, with the shower opposite.

Another option would be going with a 'wet room' concept where virtually the entire room is a waterproof shower area - so no shower doors. The sink could be very small and located in the shower. Hard to explain, but Google 'wet room' and you'll see what I mean.

I just want to mention, that for woman, having enough width to the room is important. I don't think people realize this but when one is attending to female hygiene at shall we say certain times of the month, having enough leg space becomes a huge issue. The toilet area in my main bathroom (very old house) has a radiator right next to it on the right side, and a wall to the left. There is literally only a few inches on either side. When I purchased the house, it seemed "ok" but once living there, I realized I could not use this bathroom at all for those times of the month. It was literally, impossible. Thankfully, we have a powder room on the first floor or I would have been in big trouble, as would any female guests.

Something to consider. 4' can suddenly disappear once you install the toilet; the walls are just a bit too close for comfort IMHO.

8 x 4 would be adequate for a small residential bathroom with shower, particularly if you made it a "wet room" where the whole floor was treated like a shower pan with drain.

Even a conventional shower of 48" x 32" at one end, a toilet at the other, and small handwashing sink in between would be adequate. The clear floor space in front of the shower and toilet can overlap and the floor space for in front of the toilet can include some of the space under a wall hung sink.

Ah, true. I was thinking that you'd be entering from one (narrow) end of the rectangle. If you enter from the middle of a long side of the rectangle, you have more configurations available in the smaller space.

I have a master bath that is 4' 6" by 8'. I'm in the process of remodeling it and I wish it were just 6" wider. While I think everything is to code, some of my challenges included that I needed either a round front or compact toilet and an 18" depth vanity. I can't see how 4' wide could work.

Better consult your local building code before you make any final decisions.

We're building an 8'x5' and it is small. The sink, toilet and shower are lined up along the long wall with the shower spanning the entire 5' at the end of the room.

The good news is you can splurge a little on flooring since the space is so small! ;)