In the 1954 movie, godzilla was a giant prehistoric reptile, reawakened by nuclear bomb tests in the Pacific. During this film, Godzilla is officially stated to be "Over 400 feet tall"...

This means that if what Legendary and Warner Bros. say is true, then this Godzilla is actually not the largest design in neither film appearance nor comic appearance. The only way that 2014's Godzilla can be what the creators say he is, would be this:

A Godzilla that ranges from 500 to 700 feet tall

this is all I can say considering it is officially stated that both creatures are

"Over 400 feet tall" 1954

"The Largest Godzilla Yet" 2014

I'm not sure if further proof is possible, just keep this bit of information in mind. It's all I got.