If you're really intent on using the Android calendar widget, I can't help you. But if you're willing to use the HTC Calendar Widget, once you choose that widget it will open up and just below top center of the screen you will see "calendar view 1 of 3" (That's from memory but I'm pretty sure it is right). By sliding your finger across the touch screen, you can scroll through three alternate views, and one of them is an agenda view. Select it near bottom center, and it will install. To my knowledge, this can only be done as a full page view, not a small view like the calendar widget, though.