I dont really use the feature, but i agree that it should be longer. Why? simply because of the max nick length on the majority of networks is about 30, so 30 plus the <?> and timestamp is conciderably larger than 25.
If you use the -i switch to create a sort of column-style look to your mirc, then you will instantly notice that nicks that are not fairly short create alignment issues.
25 minus the timestamp length, minus the length of the prefix and suffix around the nick ends up giving you only about half of that indent limit left for the nickname. So with a basic format of [xx:xx?m] <@nick> if nick exceeds about 12 or so charactors then the first line will pretrude and create an undesirable look.