Remember tile is permanent so you would be wise to select something which will not go out of style or be difficult to design around. Earth tones will always be good.

That said, I cannot imagine selecting floors independent of the walls. Size of tile is somewhat dependent upon the space. 12" works almost everywhere. Smaller tiles with more grout lines make for better foot security - especially if you will be using polished stone. Personally I believe glass tile is trendy though they look nice. Glass is more costly to install.

Make a drawing of your space - floors and walls, then visit a quality tile shop. Look at their displays and purchase a few tiles to take home (they're returnable), then speak with one of the deisgners. Generally a free service when purchasing. They will assist with borders or accent inserts to liven the space. You might combine stone with ceramic or go all the same.

Once you've selected the tile, move on to paint. My 2 cents. Good luck.

The style you want; contemporary, eclectic, traditional, etc. will have a lot to do with your choice of tile. If you want a very modern look you will probably not want to use a lot of trim pieces, so you will need a natural stone or a through body porcelain, but if you are going for a very classic look you might want to look for tile that if available with trim pieces, bullnose, pencil liner, etc. I agree that talking to a designer in a tile store is the way to go, along with your drawings you might take along some photos of your bathrooms. Make sure you look at tiles under various lighting, beige can have a lot of pink or yellow that might not be obvious in the showroom.

More people are choosing to remodel their bathtub and shower in a modern bathroom design, rather than settling for the basic or vintage look. These homeowners are now upgrading and adding extra space, extra personality, or extra storage with a bathroom remodel.