In Chicago, legal blade lenght is 2" if under 18 and 2.5" over 18
There are quite a few knives 2.5" and under that can be adequately employed for self defence. In jurisdictions with 2.5" limits I have carried karambits with blades under 2.5", A hawg hawkbill at 2.25", a CAS Iberia Citizen 3 position knife with a one armed bandit thumb stud, and a few other knives I have that would fit the bill. I try to know the laws of the jurisdiction and carry accordingly. That said, you do have to be aware also in some jurisdictions what is viewed as a dangerous weapon, thus a karambit may seem dangerous or in California, the Citizen would be judged a push knife. Some places you can wear a belt buckle knife like a Bowen and in others it is illegal to do so. Knowledge of the law often isn't enough, how may be interpreted in a particular jurisdiction is important. But, consider, once a knife is brandished or actually used, it often becomes a dangerous weapon in the eyes of many in LE.
I almost always carry a tactical pen, one that does't look like an ice pick or look designed to use as a penetrating stabbing weapon, Such as a Schrade, LCP lightfoot, Hinderer extreme duty, etc.; these can be as quick and devistating as a knife but have a casual appearance.
Many knives I select to carry have a couple of characteristics, one is potential for retention, thus a karambit or the Citizen, and the other a potential to use when folded as a kubotan like a Microtech Kestral.
I believe in options to meet threats be the appropriate level of force avoiding escalating to deadly force if not needed.
So much for some of my carry options. As for the best strategy for self defence; not put yourself in a situation that calls for it. I grew up in a country and place where knife fights were frequent and have the scars that go along with that and it taught me, the best knife fight is the one that never happens. I also found that brandishing a weapon hoping to scare someone often is the trigger for escalation. In this country you had best be protecting yourself or someone else from great bodily harm or immediate death, unconcerned about the follow on ramifications because once you use a weapon and harm another, even in what might seem a justifiable situation, you will likely end up with consequences that will be life changing...and not in a good way.
Aside from the Books mentioned by SharpJax, the following link is worth reading.
http://www.nononsenseselfdefense.com/knifedueling.htm