Taking a wild guess, I added the following lines to the com.apple.Safari.plist file located in the User preferences folder:
<key>WebKitMinimumFixedFontSize</key>To my surprise, it worked, and Safari will no longer display fonts at any sizes lower than those specified.
<integer>13</integer>
<key>WebKitMinimumFontSize</key>
<integer>14</integer>

