Late winter or early spring is a great time . October may not be too bad. I may wait until the temps are consistently cold.
You could prune to 1/2 the size and have no problems.

Enjoy, Mike

Here is a link that might be useful: Garden Design Ideas

I would wait until they go dormant. I usually like to wait until early march when new buds start to appear. You can whack them back to a foot.. That will keep them compact to about 3 feet by the end of the year... 18" will get you 4 feet. They will bloom every six weeks from may-November.. You can take off 6" after each bloom cycle and they will pop even better... 6-8" is about the extent of "pruning" from April- dec. hard pruning is best jan-march.

I prune mine late Feb-early March. This year I slaughtered my pink one-- to like 8' inches off the ground. It rebounded joyfully to 3 or 4 feet. There is an amusing You Tube video of a well-heeled gentleman showing how to cut your Knock Out back. Note the bottle of beer in his wheelbarrow.