Very simple to use. In your browser, right-click on your bookmarks toolbar and choose the New Bookmark option (this was tested under Firefox 3.5, but should also work in Safari - not sure about IE). Name it whatever you'd like, then for the location paste in the following code:
javascript:window.alert("Selection size is "+document.getSelection().length+" characters.");
In Internet Explorer, use this code instead (same steps apply):
javascript:window.alert("Selection length is "+document.selection.createRange().text.length+" characters.");
Save the bookmark.
To use, simply highlight the entire writeup you're checking (note that this only applies to a posted writeup, not the text entry box for a new post), then click on the bookmark in the toolbar. A window will pop up giving the length of the highlighted area.
I leave it to you folks to determine how many characters is appropriate for a writeup.