The wand is as big or small as it needs to be. The size can vary a lot, even between shots in the same scene, especially in Season 1.
As a general rule, I think the most "correct" proportion relative to Star would have the length of the grip be about as long as her forearm, and all other proportions of the wand's design following from that. It's definitely intended to be comfortably used with two hands, and the bell is large enough that it can used as a makeshift club. Maybe roughly the size of a large orange.
Really this isn't something to be worried about too much, especially the size relative to Star's face since the show's style really exaggerates heads and mouths.
For what it's worth, the wand is magical and can shapeshift easily, and apparently fits in Star's little purse which it is clearly and consistently larger than.