I have quilted a double bed size on my singer using its ordinary foot, and just did SITD. Even that was difficult because of the small area in which to work. Doing the quilting in sections works best, and starting in the center helps keep things straight and in line.
I am fortunate to have a sister who owns a long arm. She can load it really well, but has a great deal of trouble getting it to sew consistently well. I have trouble loading it, but can get it to sew pretty well most of the time. Together we are a good team! On the long arm, I have quilted a king-size for my son and his wife (pictured in my avatar), a single size for my daughter, and just tonight finished a queen-size for a friend.