https://www.flowma.com/ultrasonic-flow-meter/ shows the operations of a non-contact (meaning the sensor stays dry) by using the dopler shift due to flow.

 

Is this for a permanently installed leak detection system that is constantly monitoring the building water system? Or is it for doing a check for leaks occasionally?
Using an under-size flow meter will certainly result in an excessive pressure drop for the rest of the system, that may not be acceptable.
There is a simple scheme, often used in industry, which would plumb the flow meter across either the existing main shutoff valve or across a second valve installed in the same line. Then the flow meter will only sense a fraction of the system flow. To know the exact value of the leakage flow will require closing the valve that the flow meter is connected across. If the second valve is motor controlled then the leak check can be completed whenever all of the individual branch flows are off, and there should thus be no flow.
(Note that there is no charge for this piece of engineering advice.)