GoLabeledLink: setting the label font

how can I set the font or height of a GoLabeledLink labeltext?

If the GoLabeledLink.MidLabel is a GoText, you can just set its GoText.FontSize (in Font points) or its GoText.Height (in document units). Setting the GoObject.Height will actually change the Font only if GoText.AutoRescales is true.
If the label of the GoLabeledLink is something more complicated than a GoText, you’ll need to find that text object first, so you can set its FontSize property.