For posting at Mudcat, I'd say that having more than about 20 characters of an URL visible, can be a problem. If at all possible, make a clickable link if the URL is any longer than 20 characters. But if you can't make a clickable link, paste the entire URL in place, and there's a good chance one of our trusty volunteers will turn it into a link.

If I want to post a link to a search in the Roud Index, I'll end up with an URL that's very long, so I sometimes abbreviate it by just giving the main address of the Website, like this:

http://library.efdss.org/cgi-bin/query.cgi?cross=off&index_roud=on

The Library of Congress American Memory Collection can be just as bad, so I put this:

http://memory.loc.gov/ammem/index.html

-Joe-