I actually don't like 6" knives. They're too wide for small bits, and not wide enough to properly feather out patch seams. My go-to set of knives is a 4" and a 10"; the 4" for scooping, stirring, and applying, and the 10" for final coats and wide swaths to properly fade in/out from seams. Occasionally, I'll want a 12" or 14", but those instances are relatively rare.