This is more since I can't recall, but is transport size usually thought to be the unit plus its support/supply or something like that?
Edit: Since I'm trying to figure out if something similar in weight to a WW2 british cruiser tank should be a medium or light vehicle
Yes, it is because INF+PW is 5 tons and most players go with the idea that a single INF is a single soldier. But don't get fixated on the tons, the system is quite flexible for RP purposes. I'm actually starting to use the following categorization:
Light Vehicle - armoured trucks, scout jeeps, armoured cars, transport/utility helicopters, and such.
Medium Vehicle - light tanks, heavy armoured cars, combat helicopters, armoured personnel carriers, and such.
Heavy Vehicle - medium tanks, infantry fighting vehicles
Super-Heavy Vehicle - heavy tanks
Ultra-Heavy Vehicle - futuristic war machines
So, your Crusader II would be a Heavy Vehicle with MAV and ACP. Remember that your armour value depends more on what level of DS armour you've researched rather than the base unit type and same goes for weapons. A WW2 tank isn't necessary any lighter than a modern tank or a futuristic tank, it just uses regular old steel instead of ceramic composite materials or exotic neutronium-duranium layers.