What stone size is this?

I want to order a ring similar to this one for my girlfriend. I contacted Mona if she could make it, but now she wants to know the size of the main stone. Does anyone have an idea, what size the stone might be?

I want to order a ring similar to this one for my girlfriend. I contacted Mona if she could make it, but now she wants to know the size of the main stone. Does anyone have an idea, what size the stone might be?