I'm going pretty strongly for 10mm.
In fact I should post a WIP as it's all I've doing today and yesterday. =-P
I currently have a mix of 4 different sources for buildings:
Ratibes: Windmill, Tower and Base, Village Well
Hovels: Gothic Church Ruin, Bridges, Barn
Total Battle Miniatures: Barn, Church, Half Timbered House, Warehouse, Stone Walls + Corners
Fenris: Ruins, Egyptian Style Ruins
And I have a mix of trees that would work for 6mm up to even 15mm, all from S&A Scenics:
http://scenics.co.uk/I also have all my hills from them. They make ones in desert or green colours, as well.
They sell area templates but I ended up making my own out of cut up MDF with Vallejo texture paste on top.