When I got my first tractor, a fellow at work told me that a brush mower could handle anything I could drive the tractor over. He was wrong: Some things I back up over to mow, I don't want to drive the tractor over it!
Your tractor will handle a 6' blade just fine. If you get into really, really heavy grass or weeds, you might have to slow down a bit but normally I zip along without any problems at all.
Enjoy your new tractor!
Ken