As you see, I have made some "BIG" tanks which have a right OBBOX and set large number to their geometries. However, the tanks will still overlap when they get close.
Why not try shriking the models and see if that fixes it. it could be the game just can't make a large enough collion field. I really don't know much but thats all i can think of.
Please don't flame me, i don't know how TW works yet.
To CCGX
As I said, the model have a suitable OBBox. And It can be found in the pics that the tank isn't bigger than the original GDI war factory.
To Mongoose
Thanks for your advice. As my tank is just a little bigger than Mammoth, I don't think it exceeds the game limit. I will try 3 ways later, which are
1. refer to the code of Mammoth, as it was copied from Predator Tank at first.
2. a much bigger OBBox
3. more than more Shape items, like War factory.