Do vehicles have to break away from standard sized characters with Plasticity?

PLASTICITY BREAKAWAY +2. Adjacent opposing characters get BREAKAWAY -2. Adjacent
opposing characters of the same size or smaller can’t use or automatically break away.


How do you determine a vehicles size? Are they all standard, or is it based on their actual base dimensions or anything else?

Thanks!

Yes. Unless there is some clarification/errata that I've missed...

Just as you determine the size of any other character. It's damage symbol.

PAC
Comprehensive Rulebook Supplement page12
So if a friendly vehicle, that is the same size or smaller, is adjacent to an opposing character with plasticity, it will need a 6 on a d6 roll to break away barring any friendly buffs to the break away roll?

That is what the rules you quoted say, yes.

Do vehicles stop when moving adjacent to an opposing character even if they dont have plasticity?

Yes. Unless they have Flight or an Improved Movement that allows them to move through squares occupied or adjacent to opposing characters without stopping.