I have three "patch" knives but use only one for actually cutting patches. The other two I use for gutting deer. Their blades are about 3 to 3 1/2 inches long. Just perfect in my opinion. They were built by a friend of mine, Bill Hunt, in Seymour. Indiana. He and his son make beautiful knives. My wife bought the first one for me as a present. It is the one I use for a patch knife...and only a patch knife. It is used for nothing else since I like for my patch knife to have a razor sharp edge to cut the patches cleanly. All three hold a razor edge beautifully. All three are beautiful knives but since I don't know how to post a picture on the forum, I guess you will just have to take my word for it.
In summation, a good patch knife for me is one with excellent steel, a blade about 3 to 3 1/2 inches long kept razor sharp and used for nothing but cutting patches. That's just my opinion for what it's worth.
Oh, and don't saw it across your muzzle when you cut your patches. You will just dull your knife quickly and damage your muzzle in the process. Cut just above your muzzle and don't let your knife edge touch the muzzle as you cut. If the side of your blade touches the muzzle, you are okay, just not the cutting edge. Watch out for those fingers holding the patch material. They don't like the cutting edge of a razor sharp patch knife. More free advice.