There is no "standard" size, so you can do whatever fits the space best. You could even make a cardboard template or two of different sizes to see which you like the best.
When it comes time to install them, choose one single size to work with. You can start your layout in the center of the wall and work your way to the corners so that the panels have even reveals on each end, or you can start in one corner and work your way around the room, breaking partial panels around corners. I personally like to lay out from the center of the wall.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!