Circumference (Measured at its maximum point.)
The first dimension is the over-the-top measurement. Using a tape measure, measure the stem end to the blossom end. The tape should touch the ground on both sides, in an upside-down U.
The second dimension is the side-to-side measurement. This is, as the name suggests, the measurement perpendicular to the OTT measurement, from side to side of your pumpkin. Again, the tape should touch the ground on both sides of your pumpkin, in an upside-down U.
The third and final dimension is the circumference of your pumpkin, in its largest dimension. In the picture above, it is parallel to the ground, near the just above the blossom.
I recorded the following dimensions this afternoon: OTT, 59", plus side-to-side, 61", plus circumference 81 1/2", rounded up to 82"=202" total, which is about 184 pounds, according to Len Stellpflug's 2005 chart. This is only an estimate because the walls of the pumpkin could be quite thick, which would mean the estimate is light, or conversely, the pumpkin walls could be very thin, which would mean the estimate is heavy. Ultimately, the only way to definitively know is to weigh your pumpkin. I hope that this example has been helpful.
Have a great evening!