It's not about limitation of characters at all, but about limitation for guests to not post an URL or web address. And - we already have such module here:" Anti guest addon " - Reducing spam - guests cannot post links
Limitation of characters is really annoying in terms of posting for guests. Guests may also post a very useful message. Here on our board we have at least 15% of useful and stored messages posted by guests. I doubt that miniBB would be useful if there would be such "limitations" introduced. I am personally against such thing, it's more seems like a limitation of free thinking than preventing spam.