Though commonly misused in place of "cojones", saying someone has "big cajones" means, literally, that they have "big boxes", which is great to say to a mover or a, uhm, box-maker-person, but isn't quite the right word when youre refering to someones' tesicular qualities.
The reason is that "caja", root of "cajones", means "box, drawer or other container" (Rand-Mcnally Pocket Spanish Dictionary, 1993 Edition), while "cojón" (root of "cojones") is a somewhat vulgar term for "testicle". | 
