That all depends on the quilting frame that will be used to quilt it. I have a 120" frame, but wouldn't be able to quilt a 120" wide quilt. Probably wouldn't try anything over 110". If you plan on sending it out, contact the long arm quilter who will be doing it for you and ask them how wide they can go. If you want it much wider, then I suggest you do it in two parts and stitch them together once they are quilted.
My first quilt was a king size quilt for my bed. It's approximately 110" wide and has a nice drop on each side. I sent it to a lady who has a Gammil Statler Stitcher and she did a wonderful job on it - but like ckcowl said, it's pricey!