I've been a floor tile sales person for many yaers in another life, and the only thing I can say is that...there are no rules. You just have to be conscious that the biggert the tiles will be , the more slipperry they will be., the joints action as an anti skid. So if you go with a bigger tile , you will definitally need a carpet of some kind beside the bath/shower. What I likes , personnaly , was the smaller type of tiles ( 2x2, 4x4) and make some type of patern: allwhite with black around , stuff like that , very classic. if you go bigger , you can put them in a dimond shape ( Now , my english is not perfect , so I hope you understant what I mean)This will give perspective to the room.
Fot the grout , you have many solution: first , choose one that countains an epoxy additif, that makes them less porous. They sell all kind of colors , but I almost always suggested some type of grey , since the joints always en up this color anyway. You can put a seasler on the grout as well. it takes a bit of patience , but worth the effort ( obviously , more work with smaller tiles )
Hope this help !