Where you are going to install drywall, it's best to have as many studs at 'standard' spacings as possible so you can use full sheets. This isn't as big a problem in a small room. Keep in mind that the paper on drywall has grain, and the proper way to install it is with the long sides horizontal. The difference is more than you might think. Makes it easier on the opposite side if you ever want to install something and are expecting that standard spacing to attach something to the wall, too.
For your shower, if you're tiling it, take a look at Kerdi at www.schluter.com and check out www.johnbridge.com for help with building it or tiling issues. If you decide to use Kerdi, you can use drywall in the entire room; the Kerdi is a totally waterproof, tileable membrane that puts the drywall behind it, so it can never get wet and is the preferred substrate for building a shower. Cement board (cbu) would otherwise be required, and the most commonly sized sheets are 3x5', so if you use them, you may want to take that into consideration when spacing your studs in the shower area. They can guide you in the many proper ways to build a shower. My favorite is Kerdi, though.