This is a correction on the term throat- not to be picky but to help in your search as you talk to people- the throat is actually the opening in the throat plate- the metal plate where the feed dogs stick up through and the throat is the opening where the needle goes down throw. The Harp- is the area of the bed that extends from the needle to the pillar- the right side columb.
Unualy a machine that has a generous bed extension to the left of the needle has a smaller harp and vis versa. 2 vintage Singers that have the largest harp are the models 66 and 201. Both have a harp masuement needle to pillar of 8 inches. Because of the plstic bodies of more modern machines many have a bulky body and in order to get even that much of a harp have to enlarge the machine which often then puts it into a new class and price catagory.