As an engineer, I did some exeperimentation to see how big of an Ornithopter (epic flying mount) I could make in Shattrath.
If you're an engineer, you can use a transporter to possibly get a size buff (seems like about a 5% chance). There are 3 growth buffs: Split-Soul: Good {you're also golden and translucent}, Split-Soul: Evil {you're red and translucent}, and Enlarge {you feel somehow larger}. I've confirmed that at least two of them stack with each other (the two split-soul effects) and they stack with Elixir of Giant Growth, Heroism/Bloodlust, and of course, Mojo. The 3rd one probably also stacks, but getting 3 of them going at once is proving difficult, especially considering there's also a possible shrink effect.
Unfortunately, I was doing this experiment with my Alliance character, so I cannot get Bom'bay's growth effect. The character is also a hunter, which means I can't use the Bulwark of Kings.
Reviewing current list of stackable growth buffs, using ModScales from WowHead, which are given as % increases (ModScale 30 = 30% = 1.3x size):
- (20m) (ModScale 25) Elixir of Giant Growth (or Winterfall Firewater)
- (2m) (ModScale 30) Heroism/Bloodlust (shaman totem)
- (5m) (ModScale 50?) Bom'bay's Grow buff (horde only) -- can be doubled with a logout
- (15sec) (ModScale 30) Bulwark of Kings (for plate-wearers) or Winterfall Firewater spell (via spell-steal)
- (1hr ea) (ModScale 20) 2x Split-soul and 1x Enlarge (?) effect from transporter items (engineer only)
- (20m) (ModScale 30) 1x Larger and Oranger from Hallow's End Candy (only during Hallow's End event)
- (15sec) /kiss Mojo (a pseudo-buff for your mount, since it sizes to maintain a constant height, which makes it much larger when you turn into a frog)
- (2m) General's Warcry, from AV BG -- almost not worth mentioning, since the queues can be longer than the other buffs, and you might not get the Warcry; but it does stack with Elixir of Giant Growth. If you had everything else going into AV, you wouldn't make it out of the cave anyway, so this is only here for completeness. Bom'bay's growth buff would likely not last long enough to get the first Warcry (especially considering queue time).
- (12 sec) (ModScale 150) The Chosen One -- You get this while fighting Terokk, about 15 seconds in, if you have the highest threat. It does stack with Elixir of Giant Strength (but I wasn't quick enough to SS the proof -- very short duration on this one).
- (15 sec) (ModScale 35) Growth Ray (unconfirmed: from a charmed/possessed Sunseeker Engineer)
- (?) Magister's Terrace boss effect (pending research)
Using only effects 1-6 with no doubling, you'd be at least 4.91x normal size (3.27x for Alliance). With all three transporter effects and two Bom'bay effects, you'd be at least 12.43x normal size (5.52x for Alliance). If you happen to time it right with a Terokk fight (and as a plate wearer, odds are good you're able to hold the aggro to get the buff), the Chosen One buff will make you ridiculously huge (an additional 2.5x, so roughly 31x for horde, 13.8x for alliance) for a dozen seconds.
So, it seems that in order to get as large as is hordely possible, you need to be a lvl 70 male Tauren engineer who wears plate (warrior or paladin or Death Knight).
Note: even if you're not a Tauren male, the largest PC, one of the transporter effects you can get turns you into a random PC race of either side (sometimes changes when you cross continents/worlds/instance barriers). So it's still remotely possible you could max out your size by randomly turning into a male Tauren. As another consideration, there's a trinket that Terokk drops that can turn you into one of 3 models of Arakokra (sp?), which are about as big as male Tauren, maybe a little bigger even.