Willow thanks you for the compliment! To my best estimation, she's part Pit Bull and part Whippet... I call her a Pippet!
Because she's a bit curled up in that photo, the sweater looks like it comes down further than it really does. It actually stops at her ribs when she's standing... definitely a safe distance from any boy equipment.
This pattern is knit from the top down, so any adjustments to the length are very easy. Willow has broad shoulders and a narrow waist, so I adjusted the pattern quite a bit. I chose the correct size for her chest circumference. Then after I'd knit down past the arm holes, I slipped her front legs through the holes and estimated the length that would fit her. Then it was just a matter of starting the decreases earlier and adjusting the stripe pattern.
If you needed to make the sweater a smaller circumference than the size Small, you could simply cast on the fewer stitches and do a little figuring about spacing the arm holes. Size Small in the book is listed as 12" long and 15" around.