I find 10mm is about right for my 6mm figures. I suspect 12mm might be better for your “Large Scale” set. I strongly recommend a heavy duty magnet base, though, and steel movement trays. You’ll find them most efficacious.
Also, re: buildings, you might look at paper terrain to get off the ground and try some various scales. The input is low and will let you see which sizes best suit your desired effect.