Thank you to you both, but... (there's always a but)
karmatashi - The div id is set in r_sidebar.php. I have tried changing this to match the div id in l_sidebar.php (l_sidebar). Result was no difference. Even renaming l_sidebar.php to r_sidebar.php resulted in the same formatting as I currently have, ruling that file out as the source of the issue. I have tried playing with allareas of the css where sans-serif appears - no result.
FrankHagan - Thanks, but making that change has an effect on the left sidebar as well. So, if I reduce the font size, although the right sidebar is reduced, so is the left. Also making a reduction to the font size in this way effects the font size on all pages.
The thing that is making this tricky is that the different font size is ONLY on the front page. It is perfect on all other pages. Checking the HTML source for the home page and another page, making a comparison reveals no difference there.
I can find no logical reason as to why the font size in the right sidebar should be different on the home page only???????
Your advice is very much appreciated, and I thank you for taking the time to look at this.