Currently, the Left Frame is not very usable when large font sizes are used or when it is resized.
This was mentioned in the Usability Report:
"Users may be confused by the appearance of the left hand frame if they try to adjust its width. R: Make sure that the legibility of the frame contents is maximised when the frame width is adjusted."
I have posted a link to the patch along with screenshots of the changes here:
http://www.elijahlofgren.com/silverstripe/patches/sidebar-fix/sidebar-fix-patch-walkthrough.html
I spent a long time trying to fix this many other ways (i.e. trying to just CSS changes), and finally found this solution which seems to be the best way to fix the problem.
I hope you find this helpful,
Elijah