Thank you all.
Several people on this thread and others have asked questions on enlarging patterns. With a pattern that is based on HSTs, like this one, the math is relatively simple. I wanted a 40" wide quilt. There are 10 subunits going across the quilt, so each finished HST should be 4". I cut squares of 5" for every HST (giving myself 1/8" for trimming).
I have done the same thing with a sawtooth star, but I worked my math in the other direction. My fabric was 40" wide (excluding the selvedges), so I cut 10" squares for my subunits, resulting in 9" HST's. By adding a 2 1/2" (cut at 3") border, I have a 41" wide quilt, so I used a 41" backing, with no piecing. Since I am using 45x54" batting, I added an inner border on the top and bottom, making the finished shape I wanted.
The idea is to break your about to be supersized block down into equal subunits and work from there.