You will need to read the technical specs of the product itself.
The choice of flowmeter (diameter, technology) will depend on the quantities to be measured (volume or flow rate and at what intervals), the min/max flow rates to be measured, allowable head loss, flow conditions, budget, desired connectivity, etc. Installation conditions will depend directly on the technology and the brand you choose.
In general, flowmeter diameters are smaller than pipe diameters.
I concur that you will have to refer to the supplier's documentation or web applications to determine sizing of flowmeters; for example, Endress + Hauser's Applicator is a tool designed to help you do just that. https://www.ca.endress.com/en/product-tools/life-cycle-management/planning-engineering-traceability/applicator-select-size-instrument
I agree with Maloyaman, except to specify that if the diameter of a meter is generally inferior to that of the pipe itself, you want to make sure the variance is not too great. At the very least, care must be taken to ensure the diameter allows for silent transit of the fluid.
Depending on flowmeter technology, you don't need to reduce the diameter of your device. If you have an electromagnetic unit, for example, there is no start or excess flow rate. It can generally measure from zero to 39 feet/sec.
Krohne has sizing tool software you can download: https://krohne.com/en/dlc/software/
Sizing a point on the network for which you want to install a meter should be the last step in the process. First, you have to identify your actual measurement needs, taking into account the required level of detail, but you also want to set your objectives and expectations in terms of peak flow needs, network monitoring, fire protection flow, single or two-way flow, theoretical or real, etc.
Put simply, be careful to determine your needs before looking at equipment options. It is better to be guided by the specifics of your operations that to retroactively try to justify based on a particular flowmeter.
Thank you for your responses!