8' works really well with my similar sized L series Kubota. I'm using a Landpride RBT4096 with skid shoes, and I wouldn't want anything smaller. I used to have a light duty Land pride 6' blade and hated it. It was useless and caused aggravation for me so I sold it.
You can always angle an 8'er to be narrower, but you can't stretch a 6 or 7' blade to be wider Technically, a 7' would be more maneuverable, but truthfully I don't see much of a difference between my old 6' and my 8' blade goes as far as maneuvering. A rear blade sticks out quite a ways regardless of how big it is, and will affect maneuverability. As you can see, suggestions will vary based on use.