Our "master" bath (the quotes are necessary) is one of the smallest full baths I have ever seen. It is less than four feet wide (44") and is about seven feet long. The shower is 32"x40", leaving just over four feet for the toilet and sink. Luckily, neither of us is claustrophobic.
We installed the Kohler Rialto toilet because it is the smallest "standard" toilet on the market (regular installation and on the shelf at Home Depot). Be forewarned: we had some issues with the Rialto. If you google it, you will see that we are not the only ones. To maintain just over 25" from back to front with a low profile tank, the flush and fill mechanisms are somewhat unusual. The water does not drain from the bottom of the tank. When the flapper lifts, water passes through a modular, L-shaped piece that also acts as the overflow. Therefore, water is always in the bottom of the tank. Not a problem (unless maybe you had sediment buildup), just strange. The problem we had was that the flapper did not sit evenly on the top of the opening into the L-shaped piece. This piece can be removed easily so we could look into the hole to the toilet and see light from where the flapper was supposed to seal. This resulted in a slow-ish leak which kind of defeats the purpose of a low-flow toilet. We went back to Home Depot for a replacement, but they did not have the exact part and the replacement flapper did not fit correctly. Kohler sent us a new flapper about a week later. Once Chris installed the replacement flapper from Kohler, everything works fine. But for a $350 toilet, I prefer that it work right out of the box.
Because of space constraints, we did not have much of a choice for toilets in this bathroom. I am happy with the appearance of the toilet (visually, it takes up very little space and has a nice shape) and it does not completely block the path to the shower. We did not have to change the plumbing (install a tank in the wall, etc), so we were able to install it ourselves. Just be ready to troubleshoot this product.