Isn't one of the issues that we should define first what the potential uses might be?

Urban sounds to me like self defense. Knives by definition aren't good SD tools in an open environment. They are more of a stealth weapon in the normal world, wouldn't really make sense to show off if you intend to use it. I do believe they had a couple of surveys done on how far somebody must be standing for another person to pull and rack a gun. In this case size doesn't really matter, you key concern will be to get close to the target.

Nature: again size is not that relevant, never really met anybody in the country carrying a huge knife unless it is a panga (I live in Africa). Actually the price of pangas went down in Nigeria after the last election, makes you want to think about what they mean with electoral support.

Summary would be that we should rather focus on smaller knives but then I think about putting a fat piece of steel between myself and some perp and I start rooting for a rapier.