[Skulemate]

No, the diameter of the tubing won't matter one bit. Once the system has reached equilibrium there would be no change in the readings (though it would take longer to reach equilibrium if the tubes are a lot larger).

To convert a measured column of water into a pressure you need to multiply the column height by the specific weight of the fluid (specific weight is simply the mass density times gravity). Note though, if you're using imperial units, you'll need to make sure that you're working in the same units.

As for relating the results to the pumps head rating... well, you really can't without using the two manometers, one on either side of the pump (that will tell you how much head the pump is producing).