I wouldn't buy anything smaller than a 9" throat machine like the Juki TL98/2010, Janome 1600, Pfaff GHQ, Brother 1500, Babylock Quilter's Choice Pro?. When you get to the center of a king size quilt--many people would baste to the end then turn the quilt around on the frame if they were quilting a pattern of say 4-5". There a a couple slightly larger ones on the market now but they are more than straight stitch only like the above--Janome Horizon 7700, Bernina 830?, and possibly others . You might also look for the Bailey machines. There are stretched domestic machines and come in 13, 15 & I believe 17" models.
http://http://www.baileyssewingcenter.com/ Another alternative is to contact Wow Quilts about having one of the 9" straight stitch machines stretched to 18".