Has it ever been an issue with raids full of Tauren using Growth buffs or effects in fights? There was plenty in the game that were able to be used in combat, and it's frankly not been much of a problem. I used to use growth effects all the time as Guardian Druid back in the day, and it was fine. However, I do recognize that Blizzard has removed a lot of growth effects in combat, or at least scaled back which ones stack so you can't stack to 12x size like you used to.
Look at the Demon Hunters demon form; with the wingspans and jumping into the air, they can clutter quite a bit of screen space for bunched-up-melee DPS, but it's never really been a problem. Most of the time you don't even need to look at your position, and raid warnings are more than enough to make sure you aren't standing in the fire.
Considering a Mech would still be larger than a Tauren, all races could use the same sized contained Mech Suit model and it would work out fine. A moderately sized Reaves would solve all of those issues.As I’ve said, the mech size limits are completely up to Blizzard. However if they view size as a problem, I could see a situation where the class is limited to smaller races. A smaller race wouldn’t require a huge model to give the effect of a big mech. Also they would have a more impactful size difference with their non-piloting brethren.