I find this classic figure of LaTeX's page geometry very helpful:

Source: http://commons.wikimedia.org/wiki/File:Latex_layout.svg
Now, I am fiddling with some beamer slides and I could really use such an overview of beamer's slide layout. Does it exist anywhere? I have not been able to find it...
For now, I need it to figure out how to get the height of a frame's body.

\textheight. You could read the manual (section 8.2.2 the sidebars): you will find some dimensions here – Lionel MANSUY Jan 31 '13 at 17:57