I found this review of the Babylock Decorator's Choice machine that says it has a harp size of 9" (needle to side arm) and 6" (up/down):
http://sewing.patternreview.com/Sewi.../Review/1/3359
If you do a search at patternreview.com you may find other reviews of both machines. Just select category "serger/sm" and put the Babylock brand in.
Are you planning to use this machine for piecing, for quilting, or both? I'm just wondering because it doesn't have a zigzag stitch which means you wouldn't be able to do invisible machine applique, satin stitch applique, etc.