Presently you have to manually set the font size to bigger values.
BTW, the edge width does not scale either. But that can be solved by applying the tube filter to turn it into a 3d object.
As for automatical scaling, I could be wrong but I don’t think you can do that without hacking the source code for vtkGridAxes3DActor as well as text/plane actors used by it. The underlying properties are simply not fully exposed to the users.