On the practical side, if you are going with that size, you are going to have a lot of grout, which will darken over time, if you use a light colored grout. So if you go that route, I'd choose a dark tile and a matching dark grout. I have light colored 2x2 tiles on my shower floor, which I like very much, but I do have to scrub the matching light colored grout to keep it looking clean.

If your shower has a standard center drain, you need to use a tile that is 2x2 or smaller so that it can be pitched to the center from all four sides. If you have a linear drain, you can use a larger format tile since it will be pitched in only one direction. With smaller sizes you do still have plenty of options- 2x2 squares, hexagons, penny rounds, pebbles, etc.

I would agree with dowagercountess that a darker grout is best from a cleaning standpoint, since there will be so much grout in that one area.

Mosaic tile is a good way to go. It avoids the ugly diagonal / triangle layouts that some installers use. I prefer platinum gray grout in most cases. Grout should be neutral in any case to make the tile look more monolithic. On important jobs, I always ask the installer to make up sample boards of the tile grouted in 3 colors. It is the only way to be sure.