If you do not have a way to mill the bottom to fit ontop of the socket you will probably need to go 2" wide (to keep the block centered on the socket(socket A anyway)) and how ever much room your mobo will fit long. Mine is only 3". That is the way i like to do it but therer are others that will let the black hang out over the socket father to get 2.5" wide. But that will also depend on your mobo. basically make it ias big as you can to fit on your board. Brad has a good link to mobo holes, he will probably pop up sooner or later with it.
The base is unknown really. No one can tell you that as every design is different. 3/16" or 1/4" is probably good to start with though.
My .02
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