A quick snap shot of what I am using to scale my models. The background grid is 10mm on the major grid line, or 5mm squares.
The vehicles are, from left to right Old Crow's Gecko Pickup, My Jeep, Battlefront's Jeep, Old Crow's Outrider and Battlefront's 3/4ton Chevy.
The vehicles here are, from left to right Old Crow's Gecko Pickup, My Fennek, Battlefront's Jeep, Old Crow's Outrider and My Wrecker. Oh and Bob the scale figure standing by his lonesome....
And here in a cluster The Gecko again, My and Battlefront's Jeeps and The Bus.
Generally I am working in 1/100th scale, with a strong leaning towards visually matching 15mm figures. Strict 1/100th tends to produce models that look a little small-ish next to figures, but I believe that is the miniatures basing effect monkeying with perception.