All box blades are not created equal. You can get a light or medium weight 6 foot blade that you could most likely use just fine, depending on your circumstances. Then you could get a industrial 6 footer that you most likely could not even come close to using to its potential. I think that a lot of people get what will cover their tracks or maybe just a little wider.
I have a 66" Rollover Gannon for my Mahindra 3215HST. I would consider it to be of a heavy medium weight build, and the 3215 handles it fine. The 3215 comes with a 60" loader bucket, so I am just a little wider than that.