3/4" thick, 1.5" wide, whatever length will accommodate the VSL (typically between 25" and 28"). Before cutting slots for the frets: 1) the surface has been finely sanded, 2) the strum hollow cut out and shaped with a bandsaw, rasps and files, 3) the underside under the frets (between the nut and the strum hollow) has been hollowed out. To remove the wood, some use a table saw, some a mortiser, some a chisel. I use a forstner bit in a drill stand: the line is a bit erratic, but that's allright. This will lighten the fretboard and allow better transmission of string vibrations.
This board is black walnut.