Unlimited Size Pets! Pets The Size Of Buildings! (Patreon)

Patreon Raven has kindly contributed the following trick which allows you keep increasing the size of your non-combat pet:

Pet biscuits ( https://www.wowhead.com/item=71153/magical-pet-biscuit  ) are in-game items purchasable with pet charms from pet vendors, they double a non-combat pet's size. They do not normally stack, ie you should not be able to make your pet larger than 100%, subsequent pet biscuits thrown to your pet have no effect. 

Raven has discovered that if two players each throw a pet biscuit at the same pet, the effect is doubled for a 200% size pet. This is definitely not intended-Blizzard fixed a previous similar trick involving the toxic wasteling pet.

According to Raven, if you can get enough players (eg a  raid group) the pet could be literally the size of a zone. 

For those who like to find their own exploits: I have never heard of this technique before, ie simultaneous application of buffs stacking. It may be worth trying with other unusual  consumables , spells and items to see if you get any interesting effects.