To do this, find and alter your QBW.INI file:

For QuickBooks versions before 2007, the QBW.INI file is in the QuickBooks program folder.

For QuickBooks 2007 and later versions, it may be in a folder in Documents and Settings named similar to C:\Documents and Settings\All Users\Application Data\Intuit\QuickBooks Enterprise Solutions 10.0 where QuickBooks Enterprise Solutions 10.0 is the QuickBooks version.

Open the QBW.INI file in notepad or similar text editor.

Find or add the [FONTS] section.

Under it, add the value FONTSIZE=11 where 11 is the desired font size.  We recommend 10, 11, 12, and perhaps 13 as the largest practical size.

Restart QuickBooks to see the effect of your change.

This will alter many fonts, but not menus or reports.  Menus use your system settings and reports have their own settings.