Ditto what Big Dave said on the width. The reason is that if you go much wider, the casing can miss the jamb. Ditto on the wall only.
I assume Dave is talking 1" taller than the base cap. I generally go
1 1/4" taller than the base cap - especially if the floors are out of level here and there. That extra 1/4 seems to make it less noticeable across the width of a door. But as Dave said, it's really up to what you think looks best.
One thing to remember - though with 5" base you shouldn't have a problem - is that your bottom door hinges have to be completely above your plinth blocks. At 5/4" thick they can interfere with the hinge barrel unless you hold the plinth way back from the jamb edge. Then you are back in the same predicament where the casing will not hit the jamb.
By "missing the jamb" I mean making it very hard to nail the casing to the jamb. What many people don't consider, including many carpenters, is that the casing is an integral, structural part of the door system. It provides nail strength in sheer to the jamb as opposed to nails in tension which do basically nothing after a few years.
The closer the casing gets to the outside of the jamb, the weaker that joint becomes - doubly so because smaller nails are required to prevent them from blowing out throuigh the face of the jamb when nailing the casing on at such an angle.
Any finish carpenter with historic restoration experience will confirm this I am sure. Like Big Dave say 1/4 wider on the plinth is usually plenty. I ocassionally go 5/16th in situations where the casing is pretty massive - but no more than that.
I just looked at your PDF. With regard to the plinths, I would make them 7/8" x 4 1/4 x 6 1/4 presuming a 5" base. I would put a 45* chamfering bit with a bearing, and run a 1/16th to 3/32nd chamfer along the top of the plinths on three sides, leaving just a hint of flat exposed where the casing sits centered on top. This will allow the paint to seal between casing and plinth so you do not get a part crack / part no crack between plinths and casings all over the house.
If you were to apply the Golden Mean to your plinth the supposed perfect size would be 4 1/4 x 6 7/8. However, that would be too tall for your base size, in my opinion, and the Golden Mean cannot apply to everything anyhow..
The true test will be when you make a mockup of each (the way you have drawn it and the way one or more of us has described) to see what looks better to YOU. We don't have to look at it, no matter what you do.
Last edited by clampman; 02-14-2007 at 08:38 PM.