I would say just import a model from hl2 for reference, so if you are doing a pistol reference it with the glock, the smg, pulse rifle, gravity gun, etc. this way you could just resize it and then delete the second model.
or just make a map in hammer, insert a combine soldier for reference, import your map to XSI with the valve addon for xsi and there you should be able to scale it to a good size.