Hello,
You asked also the Netherlands.
Very simple, if it is a tool, it is legal.
Folding blades: no size but :
But the police can consider an SAK as a weapon, because it has a spring which keeps the blade from folding back. Even for a screwdriver in a club, you will get arrested.
However, a SAK won't get you in trouble, an Endura will. Basic rule: small and harmless looking.
Fixed blades:
Only allowed when you can prove you need it, in hunting trips, camping, pathfinders ....
Stricly forbidden are double edged knives and knives with their point in the center, in short, knives which look like stabbing weapons.
Actually, it is easy in the Netherlands: you can own anything with an edge, but carry nothing unless you got a good reason.
It's even worse in Belgium: You can't even own something double edged, and you can't carry anything which can be used as a weapon, so any blade locking mechanism (even SAK's) is banned. I can't find any knife fitting these criteria, but I have carried my endura for three years, I have been pulled over a couple times, as long as you don't show it off, you won't get in trouble as the police can't frisk you without proper cause. Seeing that you carry a knife is a proper cause, but I always wear a long T-shirt till over my belt, covering any clip in my pocket. Has worked for me for a ong time.
If you plan on coming to the Netherlands, don't take any knife except a multitool or an SAK, or you can loose it in customs. They already took my Spyderco rescue.
greetz, Bart.