If you aren't using "Auto Code Wrapping" and if "Code Wrap Length" is large (or blank so you get the default of 70) then the width of the main body of the monastery gates is determined by how how long the lines of code are in nodes currently on the front page.

What you say here is incorrect. Based on your later statements, I think you meant to s/aren't/are/ and are just a little inaccurate on the last part (it isn't the length of lines but the length of unbroken runs of non-whitespace characters, roughly).

The most common reasons for pages rendering too wide are:

1. Someone put a long line inside of a <pre> tag. This will affect everyone. /msg the author or solicit one of the janitors to fix the problem, usually by replacing the <pre> tags with <code> tags.
2. Someone put a really long string containing no whitespace in their node. This will affect everyone, though some browsers wrap on certain non-whitespace characters (and one browser, IE, is crazy enough to not wrap on certain types of whitespace such as spaces followed by slashes). Similar procedure for requesting a fix, which would probably be putting the really long string into <code> tags with a newline.
3. The viewing monk has "Auto Code Wrap" turned on, some code in a node contains a long string that contains no whitespace, and the viewing monk is either using a browser that doesn't support soft hyphen or has "Code Wrap Length" set fairly high. The most common reason used to be that the browser doesn't support soft hyphen (for quite a while, only IE supported soft hyphen, but now many of the newest browser releases support it).

   This will only affect the monk who has their settings misconfigured. If you see a very wide node and report it in the chatterbox and others don't see the problem, then you should check for this problem and probably turn off "Auto Code Wrap" (or upgrade your browser or set your code wrap width to something likely between 20 and 40).

4. The viewing monk simply has code wrapping disabled (and a node contains code that contains long lines). Like the previous item, this will only affect the monk with these settings so check yours (and turn code wrapping back on).

Clean this up and it might make a nice "faqlet" (site documentation node) on "Why does the page render so wide?" (or whatever).