My piggies' bowls are about 6 inches across and work perfect
Decently heavy or weighted ones are definitely a must in my expierance--- (broken link removed) because they love to stand on the rim while they eat ( it looks adorably dignified! ) and (broken link removed) they love to flip it if they can and thus spill all their food. I learned this when we got them a shallow silver bowl that was too light--- Robin Hood likes to literally play frisbee with it! lol
They have a hay rack for their hay, but I also put a handful on the ground so they can snuffle around and play in it. I usually just put their veggies/greens on the ground, or sometimes in little bowls ( like the shallow silver one ) if it's something chopped up.
I'd definitely get a water bottle over a water bowl. That way the water stays clean and they can't spill it...But the biggest reason is most piggies have water bottles from the time they're born, and they might not even know how to drink from a normal bowl.
And two of everything is always a ideal because piggies will frequently squabble over things when someone wakes up on the wrong side of the bed