Maybe you could contact the book publisher and request some information where to go to get information for enlarging the design, from designer or the publisher, and including the fabric requirements. Maybe they can get the information from the designer, and maybe in the future they will ask designers for this information. Also does the designer have a website or contact information? perhaps contacting them will get some information needed. I agree a book should have multiple sizes for quilts in a book unless it is on the cover that they are "__" size.
As to why size that only fit on the lap/legs my thought is that maybe they were designed for wheel chair users.