I did straight-line quilting on a baby blanket with no extension on my machine. I just moved the machine as far over to the right of the table I was working on. I just measured the table: 4' x 29 1/2". (I have no dining room, so not even a dining room table.)
Another time, I did straight-line quilting on a quilt 3 1/2' x 6'. I placed the machine on a small table, and put that table next to a bed, so the quilt could sit comfortably on the bed. For now, that's probably the largest I can muster, do to my space issues, but more due to my lack of sewing experience at this time.
I'm interested to see others' comments here. Living in NYC, space can be hard to come by. We have to get very inventive in the use of space.