Would have been some post about academy players but I've no idea how to find it.
It wasn't anything of note, from what I remember it was kind of obvious things to look for when you see a youth player in the U21s and what things to watch out for that might not lead me to assuming "Obviously star quality".
1. The big size thing was one of them, as you said, some youth players dominate because they are big, and while they could continue like that in a men's league, it's atleast something that has me on "pause".
2. In the opposite direction is too small. Brahim Diaz and Angel Gomes were the examples I recall where both of those players did very well at academy level, and still well at U21 level, but I always had this "pause" about them because unless they grew to be just a little bigger, I had some doubts about "star quality", not that they can't, just that the chances go against them the smaller they are, these guys are smaller than David Silva mind you.
3. Decision making. I've seen so many youth not fail the first 2 criteria on size, be the greatest dribblers at their level, but then I look at their decision making and have "pause" about them, again, they can still make it at a high level (especially if their technical ability is just super high like Vinicius Junior), but this is just another indicator that makes it harder for me to say these players are nailed on future stars.
Based on these criteria I knew immediately that Jadon Sancho and Phil Foden would be world class players, at 16, they dominated U21 level, not because they were big (and they both grew), but because of sheer technical talent and decision making that you can't teach.
I see alot of similarities in passing the 3 criteria in many of these U21 players, they just showed them a year or two later than Sancho or Foden did which is not that big a deal imo (I just think those 2 have the highest ceiling).