In most word processing programs you can select a numerical font size e.g 12 point. Why doest the CMS use the same convention?
While it may take a little getting used to how big "large", "medium" or "small" are, it means that this formatting will maintain it's relative sizes while perhaps being overall increased or decreased in size by users. Ultimately meaning a wider audience as those viewers with poorer eyesight will view a nicely formatted page.
If we all had identical browsers, screen resolutions and 20/20 vision, specifying a numerical point font size would work nicely. However there is great variation in all of these, so web standards have been developed so that users have control over the way a page is viewed. While designers may find this a bit hard to accept, these standards enable those with poor eyesight to increase the font size of the page (without affecting the graphics size).
We also note that the way Internet Explorer codes headings is much better than Firefox and Safari. Please use IE for "coding your page"