Surprised to see 6 footer most popular. I think this really depends on what you are doing with it and the location you are at. Where I live we get frost, it was under an ice age so rocks and rock and more rocks is what you get when you dig stuff up. Clay is another issue as well, I have a lot of it on my property and stuff is sticky. So sometimes I feel like bigger isn't always better. I believe the most important part at minimal is to cover your track width and that's why I feel like the 66" box blades are the ideal size for the "small" L series. The large frame L series like the 4000 model a 72" would be better, perhaps bigger.
But it really depends on what you're doing. Just pulling really light pea gravel stuff then you can get away with a 6 footer or maybe a 7 footer but the more material in the box, the more weight the tractor has to pull and eventually just going to stop it in its track.
I'm still a newbie so I'm learning...take what I say with a grain of salt haha,