My house has 8 foot ceilings. I plan to install window and door casing
that is 3" wide and 1" thick at the point farthest from the edge of
the jamb and about 3/16" thick at the casing edge closest to the jamb
edge.I plan to install baseboard that is 4 5/8" or 4 3/4" tall and 13/16" thick
at its thickest point. I would like to use plinth blocks at the
junction of the casing and baseboard and rosette blocks at he junction
of the vertical and horizontal casings.  How much taller and wider
than the casing should the plinth block be? What size should the
reveal (of the jamb) be in relation to the plinth or should the plinth
be flush with the jamb?  Should the casing be centered on the plinth? 
Regarding the rosette blocks, should the block be the same width as
the plinth block? Should the lower insidecorner of the rosette block
be aligned with the exact corner of the jamb or set away from the jamb
corner?  What size should the  horizontal reveal be between the casing
and the jamb?

Plinth blocks are usually sized in proportion to the baseboard and
casing. Width can be the same as your casing, however it is more
common to make it 1/4" wider. The height of the plinth is more
personal preference. It should be tall enough that any decorative top
edge begins above the top of the baseboard. It could be 1/4" to 2"
above depending on the style you want to achieve. Victorian woodwork
has specific style guides yet the 'rules' are difficult to find. The
block should be 1/8" to 1/4" thicker than the thickest board so that
it stands out from them. (if there is a routered or beveled edge on
the blocks account for this in your thickness calculations) The goal
is to have your trim butt into the blocks sides achieving a nice
finished look. If your Plinths have a bulls-eye or other design it is
usually centered width-wise and rises to the top part of the block. To
understand this a picture is worth a thousand words. try a google
image search.  Rosettes and plinths should be the same width.
The jamb reveal is a matter of preference but choosing a size 1/4" to
3/8" and keeping it consistant is within the norm. Rosettes and blocks
are often flush to the casing on the jamb side which pushes them out
wider than the casing and base at the joints and adds to their
decorative function. Look for millwork shops can create custom blocks
if you can't find the size you need.