Works with every race except gnomes, because, you're already a gnome.
Just today Operation: Gnomer went onto live servers.
The item you need (Gnomeregan Pride - Item - World of Warcraft) transforms you into a gnome.
Basically you're going to make yourself look big because you have a mount fit for a gnome not a [insert race here].
How to do it:
1. Use the item.
2. Mount up onto the mount of your choosing. (I used a deathcharger for this)
3. Wait 30 minutes for the debuff to wear off (I haven't tried clicking the debuff off yet. The 4hr cooldown restricts my testing a bit)
4. Enjoy looking big on a mount fit for a gnome
Warning: This affect goes away after getting off your mount.