[Horrible]

well you only can edit object size when it attach on player with this command

SetPlayerAttachedObject

honestly it posible to change size of an object on vehicle but you must use SetPlayerAttachedObject

then destroy it when get out from the vehicle with this
RemovePlayerAttachedObject

wiki link
https://sampwiki.blast.hk/wiki/SetPlayerAttachedObject
https://sampwiki.blast.hk/wiki/RemovePlayerAttachedObject