The old Usenet convention from days gone by was (and maybe still is) no more than four lines, and not to exceed half the length of the content portion of the post.

Unfortunately, Usenet conventions seem to have largely gone out the window along with the moratorium on SPAM. The folks at comp.lang.perl.misc have managed to browbeat their point across, but still consume a lot of bandwidth reminding people to follow the conventions.

I have to agree with diotalevi, that long .sigs are annoying. I guess they're not annoying enough for me to bother turning them off in my user settings, but for those who wish, that is an option, IIRC.

Update: I see that it isn't a specific option in user settings though a CSS could be used and put in place via user settings to prevent the display of sigs.