Growers have debated for years how many square feet they should let their pumpkin plants grow in order to obtain the biggest pumpkin. I believe the answer to that depends on the weather, seed genetics and your growing area. World record holder Ron Wallace reported that he increased the size of his plants some last year because of foaming stump problems in the past and when he did so he saw less foaming stumps. We don't have enough humidity in Colorado to have foaming stump issues so our pumpkin plant sizes are more environmentally dictated.
I believe to grow a big pumpkin you need a minimum of 400 square feet. However if you are going to grow on a plant that small you have to keep it meticulously maintained to grow a big pumpkin. LongmontPete is a good example of that growing a 1,306 pound pumpkin two years ago on maybe 350 sq feet. The ideal size in Colorado I think is around 600-650 square feet. Some of the ideal size will depend on the genetics of a plant (some plants just want to grow) and the weather that season however.
Last year my plant didn't want to grow. After the fruit the vine hardly grew at all and I never was able to fill in my total growing area. Some of that I believe was genetics and a lot of it was due to a super hot summer. I saw some of the same growth pattern this year with the 1421 plant but it grew more vigorously after the fruit once the vines rooted and everything is filled in now.
For the first time I measured my growing area today and come up with 640 square feet for the 1421 plant. That is more growing area than I guessed I had. The main vine is approximately 46-49 feet long right now and is still growing.