I think a gentleman's knife should be 3 inches or less. How much less depends on the man. I am a large man, and am still seeking a "favorite" gents knife. I tried a Reeve Umfaan, but it was just too small for my hand. Right now, a 2 7/8" blade seems to be the best.
What's also important (to me) is how a gent's knife is designed to be carried. I prefer a pocket clip, would like to see more clips designed for this particular purpose; i.e., clip carry in a dark suit. Perhaps a blued damascus clip (& bolster) would do the job. Traditional "steel" clips and bolsters are just too noticeable in dark suits. If you carry a gentleman's knife, I think it should be more subdued.
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"You can bet your last nickel that women is fickle..."
-Popeye, the sailorman