For a look at why variable sized characters may not be desirable lets remember Halo 2. I know I personally got great pleasure from sniping Elites in PvP because their heads were great big ol' watermelons. It was so obviously easier to snipe elites that people interested in competitive PvP pretty much stopped using them altogether. From this observation I conclude that if we can change our character sizes then anyone serious about PvP will chose the smallest size possible to reduce their surface area available to be hit. Anyone who makes a great big guardian will be that much easier to kill.

Its nice to have as it adds more to the variety pool. However, I don't think we'll see it because of hit boxes (the area where you can take damage from bullets). Tall fat players would have a harder time dodging then small skinny players for example. It can be used in games like SWTOR and Skyrim because attacks have a larger area of impact or are slow enough to dodge. Bullets travel very close to real life, instantaneously so that difference of a couple of millimeters really makes the difference in a big firefight.

Destiny is an FPS. From a sandbox balancing perspective, the last thing you want is people messing with the size of their hitbox.