Should the tile on shower walls be bigger than bathroom floor tiles?
There are no real rules regarding tile sizes. Ultimately, the tile on the floor should be less slippery, and if you do a standard drain in the shower you will have fewer cuts if you use something that is 4" or smaller. Linear drains have really opened up the possibility of using larger tiles on the shower floor because you can now slope the floor in one direction instead of from all four corners.
As far as aesthetics, go with your gut. When you are looking at tiles in the showrooms and on displays and even on Houzz, do some comparisons to see what you ultimately like. For instance, if you are doing a traditional bathroom, do you like a smaller "octagon and dot" or a "basket weave" pattern on the floor with a rectangular "subway tile" on the walls? Or do you prefer a larger square tile on the floor set in a diagonal pattern with the subway tile instead?
Helpful hint: for a small bathroom, try to avoid using a tile that is approximately 12" x 12". Whether you are conscious of it or not, it is a size we are all accustom to; for example most of us look at and hold 8.5 x 11 paper daily and it is almost the same size, it becomes easy to tell and feel if a room is small. I recommend using larger tiles like 12" x 24", 18" x 18" or smaller tiles like the "basket-weave" pattern in this small bathroom in Berkeley or even simply having the 12" x 12" tile installed on a diagonal or off-set in a "cobblestone" pattern.