How can I make my silkworms grow really really big the biggest I get them is 5cm then they spin so how can I make them grow bigger?

I know nothing about Silkworms, so is that even possible? "Getting" them to grow bigger?

How big is "really big"?

Bigger than normal.

Nature will take it's course. A silkworm needs a certain amount of nutrition to produce a certain amount of silk. And we cant regulate that. Each worm differs. Also, a silkworm eat 24/7. If they don't get food for more than 2 days they start to spin. So try and keep feeding them constantly. But as I said, if they come to a point where they have had enough they will spin.

Exactly as Bonzai said, they only eat to a point and then start spinning.
All my Silk worms have started to spin all ready, and they got feed 4 times a day or when ever some one walked past just added a hand full of leaves.

I do not think you will get them to grow much bigger than they normally grow, you can just feed them none stop.