In a very general sense there are two basic types of positive displacement water meters ( roughly 5/8 " to 2" chamber size )1) Nutating Disc, and 2)Oscillating piston. When sizing the water meter , the first # refers to chamber size, the second # to end fitting size. I.E ' 5/8 x 3/4' has a 5/8 " measuring chamber and 3/4" end fittings .
A very general rule of thumb is to size the measuring chamber not larger than one size less than the service: 3/4 service means not more than 5/8 chamber size, 1 inch service means not more than 3/4 chamber size, 2 inch service means 1 and 1/2 " chamber size.
A calcualtion based on fixture units is more accurate-usually done on larger than 3/4 chamber sized loads. Most drawings on larger buildings tend to oversize the meter-overly large chamber will cause revenue loss on low flows ( 5/8 low flow is 1/4 gallon per minute, a 2 " PD meter begins to accurately measure at 2USGM , 1 1/2" at minimal flow 1 gpm, a fixture unit calculation will increase utility revenue .
1) Nutating disc advantages: a) slightly lower head loss b) more resistant to water hammer c) more resistant to chamber clogging d) sometimes cheaper
2) Oscillating piston advantages: a) can be installed vertically or horizontially-nutating disc will prematurely wear/seize disc if instaled vertically b) minor accuracy increase at low flows .
In my area, out of the three " big" manufacturers , Neptune, Invynsens ( sensus) and ABB , Neptune alleges to be the only one whose brass meets some future proposed Lead in-water rule -don't know about this.
Suppliers link with some flow characteristics , AMR links in there somewhere:
Example: Kent/ ABB ( for PD flow charts, not that I say one supplier is better or worse):