Real World Scale

I've been working on some renderings using architectual models for my companies new website, and thought i'd put in some Ivy's.

The problem is that Ivy'gen is based upon real world scale, and a leaf that is bigger than my small town isnt right. on the other hand a 400 leaf ivy the size of a bike is too small. I don't want to start fiddling with Ivy'gen's settings (although a simple "scale" slider would be grand), so i need to scale the model.

Second problem, my scene is already 1,2 mill poly's. And with the wire-duplicate, its 2,4 million polygons. Its slow but doable.

Anyone have a suggestion on finding the real world scale of a scene?