By RAW, can a Diminutive creature use a Medium-sized Wand? Can a Huge creature use said Wand?
A magic device doesn't have a size attribute, any creature who can carry it can use it.
It may have dimensions and weight, but these don't affect your ability to use it, only your ability to lift/carry/move/stow/etc it. Arguably, even if a wand was too heavy for you to lift, you could still use it by holding it on the ground.
If the designers intended for size to matter with magic devices, they would have descriptors and tables for what would change, much like the size attribute on weapons.
There is no RAW for this, this is a case of "Common Sense" rules - nothing would reasonably restrict a creature from using any size magic device, except weight.
For reference, the page that describes a wand's physical characteristics says:
Physical Description: A wand is 6 to 12 inches long, 1/4 inch thick, and weighs no more than 1 ounce. Most wands are wood, but some are bone, metal, or even crystal. A typical wand has Armor Class 7, 5 hit points, hardness 5, and a break DC of 16.