The lowest ranked teams in terms of height in the PL are Manchester city, Bournemouth, Everton, cardiff and Liverpool. United is the 5th tallest in the league. Afcourse thats just general, in specific positions that might be different. More specificly Japan has an average height of 178.3 cm, city 179.7 and united 183.5 Whereas Watford is the tallest at 185.4
Logic i guess would dictate to use Fellaini in a more defensive function against Watford to vacuum aerial balls out of the box, and in a more offensive role against city or Liverpool and then possibly as sub, just to give a few example's. He might be considered as starter against the weaker teams of lower height.
We still have to take into account the specific players that are part of the defense as they might individually not lack in size, however the lack in height is often with the fullback in these cases. More balls to the far post then.
Things is, imho Fellaini can easily be usefull even against top teams if the situation calls for it. He's beyond class ranking imho in that he's purely a situational weapon rather than he would have a certain level at which he would fit.