The specific width that's best for you will be determined by your body – your shoulder width in particular – and riding style. For trail and aggressive riding a wider bar gives you a more stable stance and greater leverage for bossing the front wheel, and also opens up your chest for easier breathing on the limit.
If you're unsure, chose a bar with handy cut lines and err towards the wide side – you can always trim down. Do note though that carbon bars need careful cutting and a mask; you do not want that dust in your lungs!
Bars are available in widths ranging from around 600mm up to well over 800mm, but 720mm-800mm is pretty commonplace.