To change Excel default font name or size, follow these steps:
1. In the Tools menu click Options.
2. When you get the Options window, click the General tab.
3. Towards the bottom of the window there is a label called “Standard font“, and to the write of it are two combo boxes (drop down) for the font name and font size. Select the font name and font size you want.
4. The next time you start Excel and whenever you create a new workbook, your selected font name and font size will be taken as a default.
Applies to Excel 2003
Related posts:

In the current version of Excel, mine is Excel for the Mac 2008 v.12.2.3, there is no Options choice in the Tools menu.
Several websites at the top of the current Google search for “font default excel” provide the same useless information for setting a default font for Excel.
I have set my Excel font as Arial Narrow 11 point for at least five years. I have never used any other font in Excel, and yet my formula calculations still come up in Century 12 point, a font I have never used or imported in any program. The annoyance is magnified by the fact that this font and point size widen the column so that when I paste an Excel file to InDesign, it no longer fits the format.
This post applies to Excel 2003.
I have created a new post for Excel 2007. I hope it will help resolve your problem. Here is the link:
http://www.exceldigest.com/myblog/2010/02/21/how-to-change-excel-2007-default-font/