Normally, I'd recommend a SAK for such an occasion. It's about as people-friendly as a knife gets, but if you don't want a slipjoint... Multi-tools also seem to draw little attention from most people, and there are several that can be operated one-handed and have a locking blade (eg. Leatherman Wave/Charge/Surge, SOG PowerAssist).
At least in my experience, blade size doesn't have as much to do with making the knife more people-friendly. I try to be fairly discrete when pulling out and deploying my knives, but even with small blades like a Dragonfly, Chive, or Flash 1, I've still gotten the "Oh my! You have a knife!" reaction from some. SAKs and traditional slipjoints don't seem to draw any attention. Similarly, I never got a second look when using the knife on my Wave, and that has a longer blade than several of the other knives I EDC.