how do you grow one large pumpkin
When you have one fertilized pumpkin, remove all the others. More experienced pumkin growers will tell you all about stopping the plant and what to feed it on, but perhaps will not give away their secrets.
The size, ultimately, will be decided by the variety you choose because, basically, genes dictate size. You can force size to an extent by, as Welshonion says, restricting the vine to one pumpkin. It lets the vine put all its resources into the one fruit. You also need to feed regularly with a fertiliser rich in potassium.
But if you're starting with a variety that's naturally on the small side anyway you'll probably only get a slightly larger version. For truly large pumpkins, you need to start with a naturally large variety like Atlantic Giant.
The huge ones are usually grown in polytunnels and on a very rich diet of well rotted horse manure and lots of water.,
LOTS of water. They grow enormous vines too. I was about to plant one in a small bed right next to my neighbour's fence when luckily somebody put me straight. Instead we planted at the bottom of the garden in the largely uncleared bit. One part became pumpkin jungle instead of nettle jungle. It was great fun.
We left a few pumpkins on the vines in case something happened to one of them.
I heard that peeing on them helps them grow, but we didn't try this. It's also a way of saving water I suppose.