the length of the rollers is the width of your quilt that you can do. not the length of the machine. if you keep the rollers short, you can only do that width, be it child, lap or twin. For years, i could quilt only to up to a twin which was fustrating until I got a new frame. I love being able to quilt a queen.
Now as to the length of the head, that determins how much area you will quilt in before having to roll again. You will lose about 3" for the take up roll. Thus, If you have a 16" head, you can quilt in 13" of space, an 18" will give you 15". Don't get a frame that holds a domestic machine. If you do, you will only have about 8" or less of quilting space that is not worth the space in the room.
If you are short on space in the room and dont want to roll your quilt so much to get it under your machine, look into a sit down model. HQ and Sunshine 16 both offer sit down versions with a 16" head. It will work like your machine but not have to roll so tight to get it under the needle. of course, with these you will still need to sandwich your quilt in some manner.