Memory-foam products are meant to release pressure on your joints for a better rest. A memory-foam pillow is contoured with a dip in the middle to support your head and shoulders while you sleep. If you push your hands down into the pillow and then release them, you can still see the impression of your hands after a few seconds, hence the term "memory" foam. If needed, you can cut a memory-foam pillow down to a smaller size.
Mark the Size
Determine the desired size of your pillow. Top the pillow you want to cut down with a pillow or pillow case whose size you like, then draw a line around it with a sharpie marker to guide your cutting. You can also use a measuring tape or ruler if you want to be more exact.
Choose Your Cutting Method
The most common way to cut foam unprofessionally is with an electric turkey carver or bread-cutting knife. If you use an electric knife, the edges will be rough because the knife is serrated. You can also use an x-acto blade, but this will take much more time. If you use an x-acto knife, it helps to hold the pillow down on either side tightly so that there is tension on the line that is to be cut. If the foam is slack, the x-acto blade will have a tougher time getting through. You can also use a band saw or Sabre saw, if you have access to one.
Your other option is taking the pillow to an upholstery shop, where they will surely have the tools available to cut the pillow to your specifications, for a fee. Measure your pillow and call the shop first with the measurements to ask about cost before you decide to go.