Thanks Walter - yep I think that may be part of the issue. Thats a v useful tutorial. I also found some tutorials from Mamoworld which have proved quite useful for this. He has an OrientWorld plugin that looks great.. Also In AE cc2015.3 they have improved the camera tracker a bit - you can set ground plane and origin on a right click.
about scaling your scene - you can (like Andrew demonstrates in the tutorial) create a 3D null and attach the camera and all the 3d layers you created through the 3D camera tracker and scale the null, then delete the null. now you can scale your layers up if needed and have more reasonable pixel measurements for everything.