The size of this shawl is easily altered by doing more repeats of the body pattern, or working with a larger yarn and needles.
Test knitting showed that the yardage required varied significantly between knitters, which is why a wide range is given. At the 31-repeat size shown, the border will use roughly half your yardage.
However, don't panic if you're a bit short. Testing also showed that the shawl border still looks good if you have to stop a couple rows before the end!