Hi there! A typical 3"x6" subway tile will pretty much work in any space, but there are a few things to consider if you want to go with a larger scale tile or smaller. Depends on how small of a room we are talking about. Also, it depends on what the scale is of other things in the room. Sink, is it a vanity or a pedestal? Are you planning to tile all walls or just a tub surround? Is the mirror large or small? Lighting, small wall mounted or recessed in ceiling? My advise would be go with the standard size 3x6. If you room is really small a larger tile will make the room look smaller. if you use a smaller tile it could look too busy(depending on grout color if you used something other than white).

I recently did over a small bathroom. I want to say it is 5' x 6.5' (I'm not there right now so can't measure). I used white subway tiles (3 x 6 - standard) with white grout. I used them throughout the bathtub area to the ceiling and in the rest of the room about 5' up the wall (higher than usual).
On the floor I used 12 x 12 crema marble tiles, an espresso vanity from restoration hardware outlet with the crema marble cut for the top. Paint - Ben Moore Muslin (1037), a great beige. White, textured shower curtain. I also got my inset stainless medicine cabinet, faucet, and tall, rectangular lights for either side of medicine cabinet from Restoration outlet. It's very clean and attractive.

I would say that in a bathroom, the larger the tile the better. Less grout lines that sometimes can be problematic for soiling.

Your eye tells you what you like and don't. In this case either one would work. Go to your tile store and look at their show room. They are apt to have both on display. My niece wanted the darker grout, so she chose the larger subway so that it didn't get too busy. She could have gone the smaller tile with the white grout. She, too, had a 5 x 7 bath. Carried it 5' high around the room and used small hexagon black and white marble floor with a marble base. Finished it off with vintage lights, pedestal and medicine cabinet.