Q.How To Keep An Autumn Joy Plant Small
If i dont want a large tall plant. Can i prune the top to keep it small?

Certified GKH Gardening Expert
Sure! You can keep them short by constant pruning at the expense of blooms. This will make it spread outward, and keep the plant smaller, but the plant may never flower.
As long as this doesn't bother you, then pruning will not harm this plant. If the blooms are what you are after, then I'm afraid you will have to find a cultivar with a shorter mature height.
It is best to choose a plant that will not exceed your height requirements, as keeping plants shorter than they are meant to be will usually come at an expense, elsewhere.
Here is an article to help you with their care:
https://www.gardeningknowhow.com/ornamental/cacti-succulents/sedum/growing-autumn-joy-sedum.htm