elle a les yeux bleus and elle a des grands yeux are correct but, can I say des yeux bleus and les grands yeux instead of them ? if I cannot, then why i cannot? thanks a lot
I think you can say both but my opinion isn't worth that much since I'm only learning. However I would have thought that it would be more usual to use "les" when talking about a persons body parts since they are inherently specific to that person.
I'd also be interested to hear what a native French speaker has to say. Sorting out when you use "des" and when to use "les" seems to be one of those ongoing tasks where you gradually build up a picture of what is correct rather than being able to memorise a simple rule.
Yes, hivemindx is correct. For the simple reason that using "des" disassociates from the persons actual body. Elle a "des yeux bleus" may be interpreted as the girl having in her possession some blue eyes.