What Size Plastic Pipe is good for caves?

I don't know of any exact sizes but I would go for something a little bigger than them so they can fit through it.

https://www.guineapigcages.com/photos/showphoto.php/photo/12300/ppuser/10402

https://www.guineapigcages.com/photos/showfull.php?photo=12936

Here are two links to my photos in the gallery. I got this tube at Home Depot in the plumbing section for about $5. It is 4 - 4.5 inches in diameter. She absolutely loves this thing. She pushes it all around her cage...it's so cute!

You'd want to go with like 4'' abs or bigger. Depends on how big your guinea pig is. Haha, for once working at a hardware store came in handy for me lol!

Sarge loves his pvc tunnels. He gets mad when Lt. Dan uses them, so he moves them closer to the pigloo. But yeah, go with 4" minimum. Sarge JUST fits perfectly (with enough room to lay down and run through it), but he's my li'l fatty. He isn't as fond of the curved ones as he is the straight ones, though, because the curved ones tend to move more when he's not expecting it.