To grow a giant pumpkin you want to pick the best on early on, cull out the rest, and let all the nutrients go to the better fruit.
Some information on this suggests you measure each pumpkin on a daily or weekly basis and see which one is growing the fastest, this may seem like a good idea, but can be a time consuming process. It is totally up to you if you want to do this, I usually keep a good visual eye on each one to see how they are growing.
It is also a good idea to figure out if you are going for size, or if you are going for a nice shape.
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