It's been a while since I updated it, but the Legislature hasn't done anything that I know about in the meantime, and I check now and then.
There is no upper limit for the length of a folder carried in its folded condition in California. The "concealed dirk or dagger" statute, Sec. 12020, expressly exempts folding knives in their folded condition
In the same statute, there is no safe harbor length or configuration (except maybe a blunt point) for a fixed blade carried discretely in California. Even a ball point pen is technically a violation, though I haven't heard of it being enforced that way.
Folders with thumb studs are specifically exempted from the "switchblade" statute, Section 653k. California courts have ruled that the statute does outlaw the sale or carrying of butterfly knives. There's a case pending now in Los Angeles County concerning a Camillus Lev-R-Lock. Switchblades with blades under two inches long are street-legal.
Schools and colleges are sharp object unfriendly under other statutes, but a pocket knife on a college campus should be OK, under at least one other poster's reading of the relevant statute.
Los Angeles and some other cities have outlawed the open carrying of knives with blades longer than three inches. So carry your full size sport-utility folder with only the clip showing, and it's arguably sort of concealed, and that's OK. Go figure.
There's a couple of "California knife law threads" over in politics, fire up the "way back" counter and peruse. At least one contains the actual text.
In some cases, a concealed thumbstud folder of 4" or even more is perfectly legal under state law yet a technical violation of a city ordinance...but the local ordinances are *infractions*. Worst case you'll lose the knife but it's not worth paying serious attention to unless you're heading into a courthouse/PD station, etc.
In cities with "extreme knife laws" like LA, Glendale, Berkeley and to a lesser degree San Francisco (rare enforcement there) I'll pack something like a Cold Steel Vaquero Grande front right pocket, or shoulder holster under a jacket.