Which reverse macro ring do i need?

[Elin (@lluniau)]

I feel a little silly. I've just ordered a reverse macro ring cheaply off eBay, but now I'm worried I may have ordered the wrong one...
What do the sizes refer to? For example, the 58mm in this example on amazon http://www.amazon.co.uk/JJC-58mm-Reversing-Canon-cameras/dp/B003PID9PW

Thanks guys!

[Carebear]

It's the diameter of your lens. On my kit lens I can see 58mm written on the lens rim when I take off the lens cap.

[Elin (@lluniau)]

Brill. Many thanks.

[Chris]

What Carebear said. Standard size ring for Canon is 58mm, for Nikon 52mm. That doesn't mean all their lenses take that size. It is just their standard.