How to measure security door?

Call the company you want to buy from and ask them. It varies widely by type of door and level of security. If you do put in security doors, don’t forget to reinforce the walls. A steel door is useless if a crook can cut his way in through the drywall with a boxknife.

This would have been an easy question to answer had they been residential doors.
Are you just looking for a security door to go over your current doors? Are you looking for one of those steel doors that are accordion style and you pull in front if the entire storefront?
What is the size of the existing door? Is it double doors?
We really need more information to answer your question correctly.