The font size setting in Enthough Canopy Mayavi mlab appears to be broken.
Neither the command:
mlab.axes.label_text_property.font_size = 12 (e.g.)
nor the direct menu font size command (advanced settings) works.
I have logged a bug report with Enthought, but it looks like a more general Mayavi bug, see
As you say this is a bug that has to do with the translation between the mayavi layer and the vtk layer. But for now you could use this attribute:
>> ax=mlab.axes()
>> ax.axes.font_factor
1.5
>> ax.axes.font_factor=2
It only produces a small increase in size, though, as it rejects values above 2.