In working on a map mod, my players head will stick up and out of the cockpit, and lean out at each turn.

Does anyone know how to fix this bug?

You need to make either the Jet bigger or the player model smaller.

Or create new animations.

Is there any field within a vehicle or biped Tag where the scale can be modified directly using Guerilla?

edit the vehicle's seat name to something else. If it's warthog driver ("W-driver"), try scorpion driver (i forget it's label), or banshee driver ("B-driver"). It's gotta be something that has enter/exit animations in the cyborg.model_animations tag. You may actually be able to rip a model_animations tag from a map that has the correct anims for the jet pilot, in which case you'd need to correctly label the pilot seat in the jet's vehicle tag
Scale can be set using a script. But even better would be to have the driver invisable by checking that flag in the vehicle's seats section

I changed it to "scorpion-driver", and he quit leaning back and forth out of the cockpit. Thanks for the recommendations.