If you want to enjoy flowers immediately, wait until the first flush of bloom subsides and then whack away. Or do it now and you'll have flowers in six weeks. They can be pruned to 3' or 4'. You can remove a fraction of the oldest canes at the base each year to promote regeneration.

Do not prune roses in fall. Early spring when first growth is sprouting is the best time.

I would wait until after they bloom, then go ahead and trim/shape. A hedge clipper will be fine.

I agree, wait for them to bloom this spring and then you can cut them back any way you like and they should be fine. After pruning them it will take about 6 weeks for them to rebloom.

My stars, what are you feeding those monsters and where do you live?? My guess is somewhere much warmer than me, since there's no way my double knockouts would ever approach that kind of height. Whatever you're doing, keep it up, as these look like very healthy happy roses, and you'll likely have a tremendous spring flush.

I agree with everyone else that you should sit back, enjoy a delightful spring flush (with an optional adult beverage of your choice), then when the show dies down trim them down to a more manageable size. Conventional wisdom says to not cut off more than 1/3 of the plant at any one pruning, so if you want them to be more like 3-4 feet you might do that in two stages and let them rest and recover in between. Of course, most of us ignore conventional wisdom when we feel like it, and most knockouts can be pruned with a chainsaw as needed without complaining at all. If you feed them some alfalfa hay during their rest period, they might bush out even wider, but I hesitate to suggest that since they already look like the Monster that Ate Cleveland. To keep them a relatively manageable size each spring, do as Michael says and cut out a few of the oldest canes (thickest and brownest) at the base.

Keep up the good work!
Cynthia