Just drag your items down off the bottom of the page.  VS .NET will see what
you are doing and create scroll bars in the IDE to allow you to work with a
larger vertical space.

This isn't a VS .NET thing.  Web pages are as tall and as wide as you make
them.  You make them taller and wider by putting content further down the
screen and further out to the right.

And, you can stack items on top of other items if you either use Grid Layout
for the pageLayout property of the page or individually use Absolute
Positioning (CSS) on the particular stacked items.