Edwin, the dimensions shown are based upon the sizes things were drawn
when I was working out the design of the cabinet. For example the base
is drawn to be 5" wide stock (by 1-1/4" milled from 2x6 construction
lumber). I controlled the lengths of the various pieces. The
dimensioning is simplt derived from the distance between the endpoints
selected when applying the dimension.

As far as putting a 12" dowel in a 6" hole, well you could place the
dowel through the piece containing the hole but it wouldn't look like a
6" dowel in a six inch hole.

You could change the dimension as you say from 3' 2" to 4' 6" but it
won't resize the cabinet. You could, however rescale the piece so that
the dimension is increased to the desired 4' 6" if you'd like.

By using grouping and layers you can draw all the individual components
so they can be edited indepently from each other.