Ok after reading this combat action im not sure i understand it very well. So for example if someone is riding a size 6 vehicle, he gets a -6 penalty?, a penalty equally to the "modifier to hit"???

No, someone would get a +20 modifier to shoot the Size(6) vehicle - see page 138 for the Size table and the modifiers to hit and for stealth.

This is what i was refering to.

Jinx:....the operator can make a Challenging (+0) Operate test with a penalty equal to the vehicle’s Size trait. If he succeeds, he negates one hit from a single attack, plus one additional hit per degree of success he scores beyond the first...

The same modifier to be hit, is the same modifier used as a penalty when attempting to avoid being hit. Being larger is so much more difficult to dodge...er jinx.

To clarify:

A Size (6) vehicle has a +20 to be hit, and a -20 to Jink.

A Size (5) vehicle would have a +10 to be hit, and a -10 to Jink.

And so on.