How to make a fully parametric cube in SolidWorks?

How can I make a fully parametric cube? I know how to make a square (using an equal relation between two sides).
I can then extrude that. How/where can I set an equal relation between the extrusion distance and the size of the sketch square?
Or is there a better approach for making a parametric cube?
2 Answers

Link values or equations see video:
http://www.youtube.com/watch?v=_NMYghOWi0I
note that the second method using equations with a global variable ("a"= ) is probably the best way to approach Part 1 of the CSWP