Stylish is awesome for that, 'though it's only really useful if you know CSS.

I have a theme for HN with this:

    @-moz-document domain("news.ycombinator.com") {
      .default {
        max-width: 45em;
        text-align: justify;
      }
    }

Solves the problem really cleanly.

I don't really go to Reddit anymore, but you can use:

    .usertext-body { max-width: 45em; }

to fix the extra large lines, and hiding the sidebar can be done with:

    .side { display: none }

By the way, this is one of the best advantages of websites over native apps: it's much easier to tame them to your needs. Even if you don't know how to program it yourself, http://userscripts.org and http://userstyles.org are incredible resources.