It totally depends on what kind of wall repair you're doing-one size does NOT fit all! Let's break it down, 'cause I've made the mistake of using the wrong size before (spoiler: mud everywhere).
First, small repairs-like nail holes from hanging pictures or tiny dents from moving furniture. For these, a 1-inch or 2-inch blade is perfect. I used a 1-inch Warner knife last week to patch a nail hole, and it was so precise I didn't get any mud on the surrounding wall. If you use a bigger knife here, you'll end up spreading mud too far and having to sand more-total waste of time. I tried a 4-inch blade on a nail hole once, and I had a 6-inch circle of mud to sand down. Never again.
Next, medium repairs-like fixing a small crack (2-4 inches long) or a bigger dent. A 3-inch or 4-inch blade is your best bet. This size lets you spread enough mud to cover the repair but still keeps it precise. I fixed a 3-inch crack in my living room wall with a 4-inch Stanley knife, and I was able to feather the edges so well it blended right into the wall. Feathering is key here-you want the mud to thin out at the edges so it doesn't look like a bump. A 3-4 inch blade gives you enough control to do that without making a mess.
Then, big repairs-like covering a large hole (6 inches or more) or feathering edges after hanging drywall. For these, go with a 5-inch or 6-inch blade. You need a bigger blade to spread a thin, even layer of mud over a large area. My neighbor used a 6-inch knife to fix a big hole in his basement wall, and he said the large blade let him cover the hole in two coats instead of four. If you use a small blade here, you'll have to do more coats, and it'll be harder to get a smooth finish.
Pro tip: Buy a set! I have a 1-inch, 3-inch, and 6-inch knife, and I use each one for different jobs. It's way easier than trying to make one size work for everything. Trust me, having the right size will make your wall repair go so much faster-and look way better.
putty knife from hanke tools
