I have labels now in my application that work fine using the usual ILabelEngineLayerProperties2. When I set them up I set the .Expression property and using the .Symbol property I set the font, style, size, and color. Is there a way that the font can resize itself based on the map's current extent? (Just asking.)
The following is not a progammatic way to accomplish but a GUI stepthrough of the concepts.
You would need to use scale dependent symbology to accomplish that.