Welcome in Realm... dig the avatar.
The reason for the limitation is that pages were taking forever to scroll through due to the unlimited height of the signatures. This is the reason many boards don't allow signatures, and I don't really care for them myself, but we do enjoy offering something that other boards do not. We certainly believe in self-expression.
My feeling is if we allow 75, someone will ask for 100. If we allow 100, someone will ask for 120.
Request noted, and perhaps someday it will change, but lets see what we can all create with the size restriction which he have now.
...and yes, the larger signatures that haven't been scaled down will soon auto-magically disappear.
Call it from Heaven Chick.