Basically, any folders are fine, concealed or open carried. No switchblades or balisongs may be carried, unless they are under 2". And if your knife does not have a "bias towards closure", or the blade sucks into the handle, then it's considered a gravity knife/switchblade. No ballistic knives, knives pretending to be something else (cane knives, buckle knives), no shurikens (any metal pointy thing with three vertexes.
For fixed blade, length does not matter, but it must be open carried.
You cannot bring a knife over 4 inches to a public building, unless it was preapproved. Not so sure about how to get approval.
However, different places have different laws. In SF, you're not allowed to loiter with a knife over 3". In LA, and I think Oakland, over 3" needs to be concealed. However, you should really check with the cities, since I'm only clear on Santa Clara's knife code. (Nothing)
Carry a sharp knife, and life will never be dull