The only wat the font size of titles can be changed is through purchasing a CSS editing upgrade and doing the work yourself. Do be aware that the size of titles on any page is a factor in SEO.
I recommend that you don’t buy the upgrade unless you have at least moderate understanding of HTML and CSS because you must be prepared to do the work on your own as there is no Staff support and very little Volunteers support.
If you are considering purchasing the upgrade, you can go to Appearance > Edit CSS. There wordpress has provided a preview function where you can try before you buy.
1. CSS is theme specific, so you cannot use the CSS stylesheets from other themes with the themes here at wordpress.COM.
2. The CSS upgrade does not allow you to change the functionality, by editing the underlying template.
3. The CSS upgrade must be renewed annually and if you do not renew the CSS editing will be gone. The theme will revert back to the original CSS.
4. This post contains important information for those who do have the CSS editing upgrade. > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/