I suggest: why not 100, 150, 200, 250, 300, 500?
I know 255 = 2^8 - 1, but does the forum software hold such a closing date? If so, which software is it that have such restriction? MySQL? PHP? Or...?
Answers: It's not a restriction of the software itself, but a curbing the programmer chose to interrupt on the table of the database within use. You might not want a user have deed signature which is five page long, but you might want the user posting a message five page long instead. The reality is, messages and signatures are handle give or take a few impossible to tell apart channel, but stored contained by different table inwardly the database(s) the forum surrounded by interview uses.
In MySQL, they probably used a "tinyblob" or "tinytext" as the string type format. it is "a Blob or Text column next to a maximum length of 255 characters." It would be a bit pointless to use "mediumtext", which is a maximum of 16777215 chars, or "longtext" beside a maximum of 4294967295 chars, because we don't want HUGE forum signatures. =]