I don't have it, but I've looked at it. It doesn't hold much at all and I'd say it's more of a wallet on a chain. I'm anxiously awaiting the St Germain BB release, which looks like a beautiful small bag.
It is comparable to any wallet on a chain regarding size. I considered buying this also, but once I tried it on I didn't like it at all. The leather is too soft for such a small bag it had no structure and I didn't feel that it would hold up.
As Havanese mentioned, I would wait and see what the St Germain BB looks like.