What size curling tong do I need?

[ICanTuckMyBoobsInMyPockets]

My hair is quite limp, collar length.
I have a curling wand but I can't do it without burning my fingers!
I want to try a wand but what size barrel do I need for medium loose waves, something like this.

Is this helpful at all?

Right, that's not helpful at all as it's come up all blurry. Sorry.

I found the pic on Pinterest. You could try doing a search on there.