In 1929 linguist Edward Sapir made up two words, mal and mil, and told 500 subjects that one of them meant “large table” and the other “small table.” When asked to tell which was which, 80 percent responded that mal meant “large table” and mil meant “small table” — suggesting that different vowels evoke different sizes.
Four years later, Stanley Newman extended the experiment to include all the vowels. He placed them in a sequence that he said English speakers associate with increasing sizes: i (as in ill), e (met), ae (hat), a (ah), u (moon), o (hole), and so on.
Interestingly, this ranking also reflects the size of the mouth shape needed to pronounce each vowel. “In other words,” writes Peter Farb in Word Play, “the psychological awareness that speakers of English have about what the vowels convey matches the anatomical means of producing them.”
(Edward Sapir, “A Study in Phonetic Symbolism,” Journal of Experimental Psychology 12:3 , 225; Stanley S. Newman, “Further Experiments in Phonetic Symbolism,” American Journal of Psychology 45:1 , 53-75.)