Blitz units - how big should everything be?

Hi,

I know that this is basically up to me, but is there a standard convention to how big everything should be?
I mean, I have a tile system (dynamically placing 3d models next to each other) and all my tiles are about 40x40 big. My models are about 10 units high. I now realize that this might be rather small - I have the camera up close so it looks normal, so it doens't really mather, but I was just wondering anyway.

Thanks.