Your CSS rule should work. I tried it on a standalone web page and the header font size increased. Please make sure that you are not specifying a font-size style for an inner element inside the header cells (e.g. the hyperlinks).
Alternatively, you can try:
div.RadGrid_Office2007 .rgHeader a
font-size: 16px !important;
font-weight: bold !important;
Generally, such issues are easily resolved with tools such as Firebug.
If you still have problems with the font size, please send us a simple demo, so that we can inspect it locally and see what's wrong. A live URL will also do the job.
All the best,
the Telerik team