Hi,
That's a Word or PowerPoint thing. The best you can do in Excel 2003 is
press Ctrl+Shift+P which puts you in the point size drop down on the toolbar,
then you can either type the value you want and press enter or press the down
or up arrows, or press F4 to open the whole list.
Unfortunately it's slightly more complicated in 2007. You can use
Ctrl+Shift+P but it opens the Format Cells dialog box on the Font tab. Or
you can use Alt+H+FS to reach the Font Size drop down on the ribbon.