Hi Jolanxbl,
Welcome to the forums and Trainz. Unfortunately, in TRS19 there is no way to change the size of the interface.
In TRS22-Plus, there's the new Surveyor 2.0 which has a much-improved surveyor with a larger menu and totally new interface. This Surveyor version is currently under development although available to Plus users only. It's usable but far from feature complete.