I have 12" wall tile, and 12 x 24 floor tile. Looks fine.

Larger tile is more difficult to install, easier to have lippage. So you'll need a professional tile installer.

Also you need a subfloor with little deflection. The builder needs to be aware of this.

The last few years we have seen larger tiles being used in bathrooms. There isn't a correct answer as each space is different and specially in a small powder room you might want to do some mockups to see if a larger tile will work out in the space. You can always cut out some cardboard pieces in the sizes you want and lay them out.

It isn't odd at all to mix different sizes. You actually want smaller tiles on the shower floor to make sure the floor isn't too slippery.

I'm remodeling my master bath and using everything from a 1-3/8" square all the way up to a 17-1/2" x 35". Have you looked on Houzz.com? They have lots of bathroom photos showing all different types and sizes of tiles.

I have 24" porcelain squares on my small bathroom floor so they only go 3 across most of the room. I love the look.

-Babka