Size, rim and nucleation point
Start with the occasion. For a reception or a toast where you fill a lot of glasses quickly, small flutes of 9.5 to 16 cl work best: you pour a small amount more often, so nobody is left with lukewarm champagne. At the table, 20 to 26 cl is the usual size, filled to about two thirds. Roomier flutes of 30 to 35 cl are meant for those who want to smell their champagne as well, because the bowl there is wide enough to let the aroma gather.
After that, watch two details. A thin, uncut rim lets the bubbles run into the mouth undisturbed. And many flutes have a nucleation point on the bottom, a minute rough spot the carbon dioxide attaches to so that one neat string of pearls forms. If you would rather have the wide, shallow model that loses its bubbles faster but lets through more aroma, those are listed separately with the champagne coupe glasses.