permitted is 11 centimetres. "Automatic" knives and daggers (knives with double cutting edges) are prohibited even if less than 11 cm. They don't define what an automatic is, but I think it would include any folder that can be be opened instantly - whatever the mechanism. Here are a couple of links in Spanish. The first one is the law on weapons, the second a newspaper article:
http://noticias.juridicas.com/base_datos/Admin/rd137-1993.html
http://www.belt.es/noticias/2004/octubre/13/armas.htm
Even if the knife is legal, should you be stopped by the police, you may have to justify your need to carry it (hunting, collectin asparagus, cooking, etc..).
Hope this helps.
Javier