If you are looking for a parametric beam that you can adjust then you
need to build one out of one of the family templates. I would suggest
starting with the generic line based template because the structural
components built into Revit act the way structural engineers want them
to. In other words in a structural plan view they do not touch the
walls they are supported on. If you build your beam using the generic
line based template you can adjust the behaviour of the model any way
you want. The family templates that are wall, floor, ceiling based
have built in parameters that make them a little less flexible. The
tutorials on families in Revit are very good, I would start with
those. Also, you may want to get Paul Aubin's book Mastering Autodesk
Revit Building. Chapter 9 is on families and is one of the longest
chapters in the book. The web site DGCAD also has very good Revit
tutorials. RevitCity has some great links to Revit blogs where you can
find a lot of great tutorials also.