HOW TO DECIDE PUNCH LENGTH AND DIE INSERT SIZE IN SHEET METAL DIE DESIGN

Punch Length will depend on a few factors, mainly.
-Thickness of strip material and size of punch , if punch is tiny in width the punch needs to be shorter so it doesnt bend or flex
-if you are using standard punches etc, idealy you would want to keep all your punches closer to stock length so less work is required to shorten them, eg standard punches lengths are generally 71mm if you use fibro. so good length would be 70mm after sharpening etc.
- punch needs to be long enough to accomodate for sharpening so you have tool life
- punch needs to penetrate through material strip to keep blank or scrap from coming up.

Die insert size would depend on folowing
-strip material thickness
-a safe rule is to have a wall thickness of minimum of double material thickness, i however prefer more, in general for thin material a minimum of 2mm wall thickness, safe is 5mm on thin material eg 1mm strip thickness,
-die plate thickness in general is better when thicker so it doesnt crack
i like keeping my die plates min thickness of 20mm on thin material eg 1mm strip thickness but tend to stay above 25mm for sharpening.

overall you must take ur press in consideration too, has it got enough stroke and daylight and then design accordingly. ive attached an image to assist my explanation.
Hope this answers your question.