I'm not sure there is an option for that. You can change the font size of the label under 'Sketch Settings' -> 'Point' -> 'Labels size' but I think that's a display font size (i.e. it is related to your screen size rather than your sketch zoom).
Probably an important question is why: either because they are taking over the survey when you zoom out (which I imagine could be very annoying) or because you want to change the 'output' size of the label (which isn't related to how it appears on screen and depends on your workflow e.g. Therion, SVG or whatever.