I'm making a few handled tools, hot cutters, punches, etc.
The material will be 1 " or 1 1/4" jackhammer bits.

I understand the overall geometry of the cutting end of the chisles.

How do I figure out the dimensions of the of the cutting edge.

Thanks

Can you be a little more specific about the measurements you`re confused about and the type of work you`re planning?
If you understand the geometry of the cutting edge then the only things left in my mind are the width of the cutting edge,which I would make wide enough to slit what I will be working and the height.
The height of the tools just needs to be tall enough to comfortably work and still leave enough material so the occasional redressing will not leave it short quickly.
Needless to say,a slitting chisel that works well for small hammer heads will fall short when trying to do things like large industrial strength gates made of heavy cross section material.

A general answer would be the tool needs to be appropriately sized to do the work at hand.
Count on making more than one slitting chisel.

Dimensions of a slitting chisel depends on what you are doing with it---slitting 2" stock to interpenetrate takes a lot bigger chisel than working with 1/4" stock.

Note that most jackhammer bits are a medium carbon steel and slitting chisels profit from being high alloy like S7 or H13 as they are generally quite thin and get buried into the hot mass.

There is an article in this month's ABANA Hammer-blows on slitting and drifting. It gives some general information on the process.

Most large jackhammer bits are 1045, the smaller ones are better quality steel.