OT but I mean, Kate's train is pretty short for Westminster Abbey (is hers the shortest one among the royal brides'?), and her tiara is pretty small (you can't find a smaller one from the vault...). Anyway the only thing that will affect her choose is her (+Harry) preference and the location, it has nothing to do with Kate. Kate is the wife of the future King doesn't mean that every royal bride after her in this generation has to be limited by her wedding choices. If BRF really emphasizes such thing then Margaret probably shouldn't wear a wedding tiara which is bigger than her sister. Regarding the train length, I guess probably around 3m which same as Sophie's, but I hope it will be longer as to me Sophie's is too short for St George's Chapel. (Sometimes I think BRF now prefers something more "lowkey", no more extremely long train like Diana's and Sarah's)
Sohpie's train would've stood out much more had she gone with a A-line or modified A-line. She already had a more fitted top, and then the underwhelming bottom part just didn't hold up. A 3M train is actually cathedral. Court train is only 1M.
I don't think it's as much as BRF prefers more lowkey, but the 80s was just a different animal.