1. goto: C:\World of Warcraft\Data\enGB
2. open locale-enGB.MPQ with MyWarcraftStudio
3. click DBFilesClient\CreatureDisplayInfo.dbc
4. use the search field to find the texture of your mount.
5. Remember the id.
6. extract CreatureDisplayInfo.dbc
7. open CreatureDisplayInfo.dbc with DBCeditor (http://files.filefront.com/DBC_Edito.../fileinfo.html)
8. now scroll down to your mount. (this is why u want to remember the id, there is no search function in DBCeditor.)
9. The number in field 4 determins the size. Dont edit the first 3 numbers, else your mount will grow way to big. i changed 1065353216 to 1069353216. (but play with the numbers, the outcome is always surprising >.<)
10. save the dbc, and put it in a MPQ like you would with a modelchange.
and your done!