From Excel 2002 help which appears in a browser window (Internet Explorer)
Change the appearance of a Help topic - Open the Help window. On the Help menu, click Microsoft Program Name Help. If the Office Assistant appears, click Options in the Assistant balloon, and clear the Use the Office Assistant check box. Click OK to close the dialog box, and on the Help menu, click Microsoft Program Name again.
In the Help window, click Options , and then click Internet Options. On the General tab, click Accessibility, and then click Ignore colors specified on Web pages. Click OK.
In the Internet Options dialog box, do one or both of the following: Change the color of the background or text in Help. On the General tab, click Colors, and then select the options you want.
Change the font in Help. On the General tab, click Fonts, and then select the options you want.
Note If you change the font and background color of the Help topics in the Help window, the change will also affect the font and background color when you view a Web page in Microsoft Internet Explorer.
I couldn`t find a way to increase the font size but have a play yourself. I may be away for a while so hopefully someone will offer a better solution.