Both shower heads have less flow after water heater replacement
First, I'd suggest removing both showerheads (and all your faucet aerators, as well, perhaps) to check for debris - major work like changing a water heater often dislodges  crud in the lines that ends up clogging the point of exit, so cleaning and backflushing is needed to restore flow. While you have them off, run the shower full bore from the bare pipes for a while to remove any additional crud you can. Don't know if I think a good pro plumber should have done this, or if the average customer would complain about the extra half and hour at plumbers rates for doing it...

As for the size of the lines, you should be able to read the size right off the side of the CPVC - possibly the copper as well, though that's a bit closer to intuitive to measure, and it's probably too old to be easily readable even if it is printed on.