Davis sells two sizes. Here's the skinny from Prodata:
Large and small: Wind cups for VP2-type anemometers have been available in two cup sizes – small and large. The 7911 (WMII-type) units were originally small cups only (in ca 2000 and before). But for many years now both anemometer types have been sold only with large cups (though small cups are still available as spares) and large is the only size that is likely to be relevant to the great majority of users. Large cups start spinning at lower wind speeds than the smaller version and for general use there is no need ever to think of using small cups. Even for anemometers originally supplied with small cups, the large cups are the best replacement.
The only circumstance in which small cups were suggested was in localities where extremely high winds (eg hurricane force – 100mph and above) may be encountered because smaller cups may perform a little better under these extreme conditions, presumably because of their lower wind resistance, though this isn’t stated explicitly anywhere. The downside of small cups is obviously their more limited performance at more moderate wind speeds.