Due to
GW's scale being "28mm Heroic" you'll find litlle that matches perfectly, even other
GW products. It's a commonly noted thing that
GW apc's, for example, are too small to hold their supposed number of troops.
1/32 is big, perhaps too big scale-wise, but do look about right as heavy/super heavy tanks, depending on model of course.
1/35 always looks ok width & height, but invariably too long.
1/48 is getting there, but is a bit narrow.
28mm (not heroic) should work, but doesn't quite.
1/56 ish, or 1/43, are common diecast metal models; cars are generally ok, but trucks are too narrow and tanks too small.
Generic "plastic toy soldiers/"army men"" vary in scales/sizes from 1/72-1/76/15mm (HO/OO SCALE),THROUGH
TO 1/32/54MM.
At the end of the day it's really down to you and generally if a thing "looks" right it is right.

1/48 hetzer with
40k gun.
[img]http://images.dakkadakka.com/gallery/2010/2/3/80523_md-samson%20%26amp%3B%20crew.jpg[img] 28mm Old Crow models SAMSON arv.
GW Chassis and Airfix 1/72 missile launcher.

Toy Soldiers Depot missile

[img] Diecast, about 1/56?

Lone Star diecast bren carrier, Sentinel driver

Airfix 1/32 halftrack
[/img]http://images.dakkadakka.com/gallery/2010/2/3/80523_md-samson%20%26amp%3B%20crew.jpg[img]