Wool is extremely sensitive to heat and moisture. Both of these elements are ideal for resizing a wool jacket. Shrinking the fibers in a wool jacket is the surest way to make the jacket smaller and more comfortable. Start the resizing process by slow increments of heat and moisture to ensure that you do not warp the fabric. This shrinking process will require testing different strategies to create the ideal fit.
Fill a spray bottle with warm water. Hang the wool jacket on a sturdy hanger and hang the garment in an open area where the jacket can dry.
Spray the entire wool jacket with the warm water. Make sure to spray underneath the armpits and behind the hood, if applicable. Wet all of the fibers as evenly as possible.
Blow dry the jacket using a blow dryer set on high heat. The heat will shrink the fibers as the jacket dries. Keep the blow dryer moving over the fabric to avoid overheating the material in any one place.
Dry the wool jacket in the dryer on a medium heat. If the blow dryer does not make the wool jacket small enough, dampen the jacket again and place in the dryer. Set on a medium to medium high heat and allow to dry for about twenty minutes. Keep an eye on the jacket to ensure that the wool is not damaged. Hang the coat up immediately after removing it from the dryer to ensure that wrinkles do not form.