This is a great question. The flow rate is much more important for meter sizing than the line size in which the meter will be installed. The quickest way to do this is what we call a bucket test. You get a bucket that has a known volume or you measure out a known volume into a bucket and mark a line. Typically you would use 1 gallon or 5 gallons as the test. Then you would run your liquid at the flow rate you are interested in measuring into this bucket and time it. If the flow meets the 1 gallon mark in 15 seconds you would know that you are flowing 4 gallons per minute (GPM) (60 seconds / 15 seconds = 4; 4 X 1 gallon = 4 Gallons). If you hit the 5 gallon mark in 15 seconds you would be flowing 20 GPM. If you hit the 1 gallon mark in 30 seconds you are running 2 GPM, 5 gallon mark at 30 seconds it would be 10GPM.
After this test you have the information you need to get the correct size meter. You normally would like to stay in the middle of the range that the meter can handle. So if you have a flow rate of 10 GPM a meter that works from 0 to 20 GPM would be great (for most meter technologies).