I've come to the conclusion that Legion flight rings have the ability to adust their own sizes.
First, there's no indication that anyone has ever asked a questions like "What size ring do you wear?"
Second, there have been occasions when flight rings have ben handed indiscriminately, as if it doesn't matter who gets which one.
Third, flight rings have been swapped in the past without any indication of sizing problems. I particular, in Superboy #213, Black Mace took Timber Wolf's flight ring and put it on his own finger. It's hard to imagine that the two of them wore the same ring size.
For all these reasons, I conclude that Legion flight rings, like the One Ring in the Lord of the Rings movies, have the ability to adust their own sizes. It's a cool bit of future tech that's been assumed in the Legion for a long time.
Of course, in Superboy #209, "Hero For a Day," th Legion gives Flynt Brojj a flight ring which is too big for him. Wildfire says, "It's a little big for his finger...but it's the smallest spar we have." Perhaps "spares" don't have all the functions of standard rings (including the auto-size option), or perhaps there' s a minimum size for flight rings, and Flynt's finger was just too small. (The fact that Violet's ring can shrink with her to microscopic size seems to argue against this interpretation—but that might be a function of Vi's power rather than any ability of the ring.)
No Legion sightings this week.