I have two Berninas, one 830 and one 155. I love them both and have them for different things. Sadly, the 830 sits a lot more than the 155, but it still gets used.
I have quilted on both of them up to a king sized quilt. (Only did one and never want to do another) I just finished quilting a smaller wall quilt, it was very dense quilting and the machine handled it beautifully. I even quilted with the 830 before we had the quilting foot. I just dropped the feed dogs, removed the presser foot and lowered the presser foot thingy even though it wasn't there, to engage the tension.
Clear as mud, I'm sure, but the point is, it's very doable. Start small and work your way up.
M