Maybe the games could be sort by the length of the shortest (or longest) full-game run (or individual level run) on the game page and series page ?

Not a patently ridiculous idea, but there's probably few series where it would be a substantial inconvenience to simply check the game pages. Aside from Mario, Zelda, Final Fantasy, Pokemon, Mega Man, Castlevania, maybe a few others, I'm not sure it would be too necessary. If a series has like 8-9 games (or, obviously, if it has fewer), just check the game pages.