Heres the official size chart for the kaiju in Godzilla US. As you might see, I didnt include all the monsters since they either dont have a colored version (Mecha-King Ghidorah) or arnt drawn yet at all (Jet Jaguar, Godzilla Jr, Titanosaurus).
Its a little halfassed on my part, but it's something neat I thought i'd whip up. I'll updated as I drawn/color the remaining kaiju. I feel like theres a slight inconsistency with the size meter thing on the side and the kaiju, but i cant quite put my finger on it.
On the chart:
Bottom row: Godzilla 1970s, Gigan
Middle Row: Godzilla, Burning Godzilla, Mechagodzilla, King Ghidorah
Top row: Grand Ghidorah