There are several buffs that increase your size.
I will list the ones I know of and tell you how to get them.
Grow - Horde only! You must have completed Zalazane before you can talk to the troll by the cauldron(can't remember his name) in Zul'Jin just outside of Valley of Trials. Talk to him and click on any of the texts(doesn't matter wich one), and continue doing it until you get the growth buff. Log out as fast as you can, and log back in. Do the same thing again (get one more grow buff) and then you have 2. You can't have 3, when you relog/change zone with 2 you will lose one. You HAVE to relog or it won't stack. Now hurry up to go to orgrimmar and show off before the buffs dissapear!
Bloodlust - Shaman spell. Just ask a shaman to join your group and cast it. It is only 30 seconds though, so you may want to take this last if you are stacking!
Big and Orange - You can get this one when eating a pumpkin when it's hallows end.
Transporter Malfunction - If you are engineer you can make trinkets that teleports you (fe. Toshleys Station/Area 52) and there is a chance to get this debuff. (1 hour)
Gnomish Shrink Ray - Gnmoish engineer item. When this thing backfires there's a chance that it makes you (and your party) big.
Winterfall Firewater - Elixir that drops from furbolgs in winterspring. Or you could buy it from ah. (20 min)
Elixir of Giant Growth - Elixir that can be made by alchemists.
The AV Buff - You get this randomly when you are in AV. Can't remember the name.
Berserking - The buff you get from the treasure chests in Eye of the Storm or Warsong Gulch.
Mojo Frog - /Kiss on a mojo pet. You will become a frog and your mount will be bigger! (15 sec)
I have no idea wich buffs that stack, but I know some don't and makes you lose a buff or two.
If you know any other buff that increases your size, please tell me in this thread and I'll add it!
This is my best: