Thanks Mohsin.
I couldn't find the exact code that you mentioned above, but in the footer template I did find something similar to it, and so I took out the "span3" references, but this didn't solve the problem.
But I posted something on Craigslist and a responder advised me to place this in the style sheet:
aside#recent-posts-4.widget-container.span3.widget_recent_entries { width: 100% }
and it worked! (BTW, I assume that the "4" refers to the max number of post titles to show, so that if I change it to "7" it will show 7 titles, when I get that many?)
Thanks again.