things to keep in mind:
make sure to take all pertinent measurements:
a) ceiling height, b) wall width, c) shower area size, toilet bowl projection (how far will the toilet bowl waste pipe will from the wall), window's height and width, shower body's position on the wall, etc. But the mos important measurement that has to be taken is the tiles that you will be using for the project.
you could be using square or rectangular wall tiles, rectangular Listello (italian for wall border), deco tiles (wall tiles with some design on it's face), matching floor tiles, only the could be (almost always) different size from the wall tiles.
at the same time you need to take into consideration where you are planning to stick these tile on the wall and floor. may be you want to lay the floor tiles on a baes (diagonal, always a winner), or just simply straight. your wall tiles, if the come with some kind of border; how are you planning to break the wall to make the border look good and not to make it interfere with other fixtures (shower body, etc). check if the window is in a good spot (not that you can or can't move it), but always a big factor when taking measurements.
remember that the first tile is the one that will take you the longest to install, the rest of the tiles are just going to follow the first one.
in every tiling project the first tile is the one that tells you where the rest of the tiles will go.