The chart was made using the following observations from the comic:
- Peanut is elbow-height (January 30, 2009)
- Grape is elbow-height (January 21, 2009) and is exactly as tall as Peanut when you don't count the ears or the tuft atop their heads (the entire Imaginate! story arc)
- Bino is waist-height (November 25, 2009)
- Tiger is waist-height, but shorter than Bino (November 25, 2009)
- Fox is somewhere in-between. He's slightly taller than Peanut (December 17, 2008) and definitely taller than Bino (July 19, 2008, July 27, 2009, and August 3, 2009)
- King is below waist-height (December 4, 2009) and his head reaches the lower part of Fox's nose (June 16, 2010)
There are two assumptions I've made that may discredit the chart entirely: Firstly, that the "vet orderly" and Mr. Sandwich are "average human"s. Secondly, that Gaspar/Pete is at least three inches taller than an average human.
And now, without further ado, the First Housepets! (Un)official Size Chart: