6-1/2" length to the fretboard, either 9/16" slot width & 1/4" sides or 1/2" slot width and 5/16" sides (depends on model), 2-7/8" to 2-29/32" wide at top and 2-3/8" to 2-1/2" width at the taper, which can occur anywhere between 1-1/4" to 1-3/8" up from the nut. 4" to 4-1/4" slot length, with at least 0.600 distance between the end of the slot and the center line of the tuner shaft. 3/4" thickness, but a little extra is nice to have.

First tuner 2" above the fretboard and the 2d and 3rd pairs at 1-3/8". The ramp should run 2/3's of the way to the nut and be around 1/2 the depth of the peghead or a little less to take the strings without contact. There are several drawings of Martin pegheads, including a clip of a shot from a martin production drawing in the Martin Guitars: Technical Reference towards the back. The S-M McRostie 000 plans has pretty good info on how to layout the neck for both 18 and 28 series (w/ and w/o volute).

As far as what I use, 6-5/8" x 2-15/16" x 2-7/16" x 4" x 3/4" x 9/16" x 1/4", or pretty close to that.