The largest I have attempted on my DSM was a twin/full size. however my DSM is not inset into a table. If it were, I think I would have been able to quilt almost any size. Why? I schrunck all my quilts rather than folding or rolling them. They are tugged and pulled as needed. The table helps with the heaviness. While the bigger harp does help, I think the inset large table is a bigger perq to me. I will admit that the stitches per minute being faster also is a large perq.
When I chose to get the HQ16 sitdown, part of the reason was to save the life of my DSM for piecing. My DSM is 16+ years old and going strong. It was the best I could afford at the time and I got it before I quit fulltime work to stay home with three kids. Now that I am back to FT work, I could afford a new machine and chose to go with a sitdown quilter rather than a new DSM. I am hoping between the two to never have to buy another machine!
And if I work out the cost of my DSM, I am looking at 20 cents per day since I've had it. I hope to have the same record on my HQ16. Right now the cost has been 2.75 per day which is less than a Starbucks coffee per day (which I also don't buy). But then again, I keep my cars for more than a decade and don't really believe in trading up to get the latest/greatest. I wanted to get the best I could afford at the time and have it last for a long time.