This video will show you how to make the measurements:
This pumpkin growth benchmark chart can give you an idea of how your pumpkin is charting as compared to pumpkins that have weighed over 1,000+ pounds. The chart is just a benchmark and particularly in the first 20 days of growth any measurement doesn't mean much. My 20 day measurement last year was two inches bigger then the former world record 1,725 pound pumpkin's measurement on the same day. That pumpkin was one of the slowest in the patch for Christy that year but around day 30 the pumpkin finally got into gear and never looked back. At about day 20 you should start seeing some increases in size and at about 28 the growth per day should be very noticeable (25-40 pounds per day would be typical from day 30 to day 40) if you have a good pumpkin.
There is an iPhone app, if you are interested, that does the weight calculations for you based on your measurements in the iTunes store.