Very few of the panels within the game UI allow you to change the size of the panels. The help menu is not one of them. The only things you can do in the Windows client are:
1) Grab it by the Handle (top center of the panel). Move the Help Panel around.
2) Use the scroll bars on the right side and along the bottom.
All I can do is suggest that write a Mac specific bug asking them to fix the scroll bars. I believe you will have to write the bug report outside of the game software using your favorite browser. Turbine may not have gotten the Mac client bug reporting system working yet.
As a interim solution fire up your favorite browser in a different window. Use your browser to access the knowledge base within the support site.
Unless stated otherwise, all content in this post is My Personal Opinion.