You can't use an entire song as a ringtone... that's why you are getting the message that it's too large. But there area couple of websites and some free apps that let you create a ringtone from a mp3 on your pc.
I can't remember the names of the sites atm. One of the programs is Audacity. I'm going to move your thread to the Media Center forum and someone there will know the websites and other apps.
Also check this for how to get ringtones on your phone.
How do I get ringtones on my BlackBerry? - BlackBerryFAQ