Just out of interest the police were out in force on the EVO triangle,due to alot of you tube clips being posted showing silly speeds !
They stopped A McLaren and a couple of BMW M3 not for speeding but illegal plates.They where letter box size!
It must be ok to stick on the wing as a number of Morgans do this but as Chesterg say six digits max ?
You have to respect the size and the space

and based on the current size you can only have 3 character on the flat part of the wing so can only be 3 + 3 Maximum , if you start to have smaller character or fancy writing i doubts you won't get trouble it's what i be told by several serious source
PS: the Normalised font is available over the web