How to link a Dimension to a Custom Properties?

I am pretty new to custom properties. We are going from SW08 right into SW2010 and I have started to mess around with the custom properties tab builder and want to set up our title blocks to pull in info from our parts.  I have gotten the basic info down because of the built in value that can pull the date, file name, and file location in.  I am struggling with getting the parts over all dimensions to be pulled in.  How can I link a dimension to a custom properties?  Is there a predefined value for the width and length of the part or is there a way I can create it?  Also in some cases I will need to add 3 dims together to get the total length, is this possible?  SolidworksDrawings And Detailing