Your blank dimensions will depend on what you want to build. Halfstock, fullstock, early, late. 2.5" width will work for almost all projects, properly laid out. Even a 2" blank will work for most guns with more narrow buttplates, barrels.
If you want to incorporate cast-off etc., you can lay out the barrel not perfectly parallel to the sides of the blank and achieve this.
i suggest getting a book or two on scratch-building. Dixons, or Peter Alexander's, etc. You might want to buy plans for your style of gun.
The two widest parts of a stock are at the cheekpiece and the lock panels. Both of those are easily calculated.
At the lock panels, the width = barrel width+ 2x thelock bolster width. You'll want an additional quarter inch for inletting and finishing.
The maximum cheekpiece width is usually about a quarter inch to 3/8 inch greater than the buttplate width on longrifles. Sometimes 1/2" greater but that is rare.
