For 12x24, I would stack.

It depends on the coloration of the tile and the look you want…If your tile is variegated (not the same color across the whole tile) and you stack it, the effect is stripes of variegated tile. If you alternate like bricks, there will just be a consistent overall look, but no stripes. If the tile is a solid color across the tile, the striping is not an issue, just a personal preference of what you like. Whatever you decide for the walls, the pattern should continue from the back wall of the shower onto the shower floor.

this is the tile i'm using (12x24). I was told that it should run length wise along the shortest wall, which means it would be perpendicular to the shower wall - is that the "correct" application or should they run the same direction (lengthwise on the long wall)? Thanks!

I am not a professional…but I would suggest look at bathroom shower photos on Houzz and see if you can find a similar showers/tile so you can see what works best and how it will look. I still think the focus wall of the shower is the long wall, and if you’re placing the tile horizontally on the walls, the tile should flow in the same way onto the floor from that long wall. If you’re not liking the look of using the same tile everywhere, a contrasting floor tile may be a better option.