Round - Dies Thread Size: 4-40 Thread Type: NC Outside Diameter: 13/16 Material: HSS Style: Split Adjustable
Stocks Holders - Dies Style: Die Stocks Die Outside Diameter: 13/16
Machinery's Handbook Toolbox Edition (Machinery's Handbook)
This book has all the information necessary to calculate the outside diameter of the part to be threaded, or you can guesstimate by measuring the outside diameter of a similar screw.
Poly-Cal Electronic Caliper-English/Metric/Fraction - 74-101-175 by Fowler - Fowler - 74-101-175
These are good entry level calipers, and probably the most commonly used tool in my toolbox.