The default amount is 15 letters. Is it possible to change this to allow more characters? Thanks.

Just putting back up top in case it's possible.

The third time's the charm?

You need to edit the __eventEditCityNameBegin() function in Assets\Python\CvEventManager.py and change the "15" to a higher number:
def __eventEditCityNameBegin(self, city, bRename):
popup = PyPopup.PyPopup(CvUtil.EventEditCityName, EventContextTypes.EVENTCONTEXT_ALL)
popup.setUserData((city.getID(), bRename))
popup.setHeaderString(localText.getText("TXT_KEY_NAME_CITY", ()))
popup.setBodyString(localText.getText("TXT_KEY_SETTLE_NEW_CITY_NAME", ()))
popup.createEditBox(city.getName())
popup.setEditBoxMaxCharCount( 15 )
popup.launch()

If you want more than 30 you'll also have to change the __eventEditCityNameApply() function below it although as you can see in the screenshot, 30 characters doesn't fit very well on the city bar so hopefully it's more than enough. Note that I've only tried this on BTS and of course standard modding caveats apply: copy the file to CustomAssets and/or make a backup before you edit it.

http://www.isarapix.org/pix90/1227946186.jpg

http://www.isarapix.org/pix78/1227946207.jpg

Thanks a bunch. I'll try it out.