If they are like the rest of Corgi's lines like that, the vehicles are sized to fit in a common box.
What I do, is figure out how long a 1/72 (compatible with 20mm...) vehicle would be in what I'm looking for, and take a tape measure with me to the store and try to measure the little buggers in the box. Then I see how close it comes to 1/72 (in this case).
There are a number of on-line sources which provide the hull length of real vehicles to use as a starting point.