Zoom in? Use a larger font? :)
What does it mean, "they appear small"? Are they old style ("lowercase") numbers? Then check if the font has (uh) "normal" numbers as an Opentype feature. If not, it's just "the font design".
If the usual ways fail, make a new character style with a larger point style or horizontal/vertical scale to your taste, and apply it to just digits using a GREP style. Nothing fancy required, a new GREP will be applied to digits by default.