I Have A Blue Aster Shrub That Has Grown Quite Large. How Far Should I Cut This Back ?

Question

Also I need to even out the shrub. Will all the flowers return on the side that I evened out ?

Answer 1

Unless it is very woody, it should die back during the winter. Then in the spring, cut back to the ground any remaining stems.
Till about July, you can pinch back the growing stems to keep the growth in check.

https://www.gardeningknowhow.com/ornamental/flowers/asters/pruning-aster-plants.htm