From Design at the Necromundicon:

GW's models are about 30 millimeters in height for the average man- and bases only make them look taller. The figures are not proportional though- everything's much chunkier, especially weapons. If you plan to use real models for anything (particularly military models), be aware that although 1/48 scale is most accurate, it is extremely hard to find- you're better off starting with 1/35 scale and selecting smaller vehicles (World War I and II tanks are smaller than modern tanks, for example). Ideally, an unarmored figure(Imperial Guard or Necromunda) should look good in one of the doors or hatches of a vehicle. You may have to replace the existing doors or hatches with your own (the hatch on the Vehicle Accessory Sprue is great for this job!). You will almost always want to replace personal weapons with 40K bolters, etc, and bulk up vehicle weapons with larger tubes to make them look right. For accessories like gas cans and oil drums, 1/35 is slightly oversized, but it looks great (and provides more cover!).