Don't start with the KPIs, start with the responsibilities.

Think of everybody in your business and list out what your performance expectations are of each person. Then determine how you will know if those expectations have been met: what are the outcomes you will track for each.
Those are your KPIs.

For example: you expect your sales people to sell. That is a fine starting point. But what does that mean?
- Do you expect new clients? And if so how many? At what value?
- Do you expect return business? How much? How often?
- Do you expect up-sells? How often?
- What activities to expect to see to lead to these results?
- How many clients do you expect to see? What percentage of business is new, versus old? Are you high value low volume or the other way around?

This will be a more effective way to find realistic measurements that are both easy to track, and easy to share and "sell" to your employees.