It has been a while since Evan Abrams has done his tutorial thing. Worry no longer, recently Evan posted a tutorial showing how to use After Effects new sourceRectAtTime expression found in After Effects CC 2014.2 (18.104.22.168) version.
The expression sourceRectAtTime can return the dimensions of text and shape layers in After Effects. Evan also talks about expressions in Ae generally, explaining how expressions and accessing properties thought after effects expressions actually works. There are all kinds of goos stuff in this one.