Comments can be 600 characters long (I assume this is the database column size), and the text is stored in Markdown format (which gets rendered on page load) - here's a SE Data Explorer query that shows the raw markdown.
It'd be technically possible to change how this works (SE could have a DB column size larger than the allowed character size, using the extra space to 'not count' formatting or 'hidden' URL characters. Whether SE wants to change this is another question.
However, in the meantime I thought I'd share some space-saving tips I've picked up when linking to other Arqade/Stack Exchange posts. Consider the following Arqade Meta question (grabbed from the browser URL bar):
That's a quite hefty 101 characters long, just over 1/6th of a comment's length. Can we do better? Well, if you use the 'Share' option on the page instead, the URL drops the title and shortens the
/questions/ path to just
/q/, bringing it to 51 characters:
But, we can do slightly better again, by using our site's shorter URL. That's
arqade.com for the public site, and
meta.arqade.com for meta:
If you don't care about earning the Announcer, Booster or Publicist badges, you can remove your user ID (and the slash) from the end as well:
who needs https? (don't worry, SE redirects to https automatically)
and apparently (according to user232393), we don't even need to explicitly list the protocol at all:
That's down to 25 characters, less than 1/4th of the original URL, and a lot more managable for writing comments.
In fact, (again, thanks to user232393!), so long as the link is to the same stack exchange site, you can drop the site name from the link entirely, and just use the path, like so:
This won't work for linking between different sites (such as the main site and meta), but it's a neat trick to eek out a little bit extra space!)
(If these sorts of little optimizations interest you, might I suggest a trip over to Code Golf? :-)