Variant: you can measure the size of a person by what makes him or her angry
Source: How to Win Friends and Influence People
“You can tell the size of a man by the size of the thing that makes him mad.”
Address to the State Committee of the Liberal Party in New York City, Faith in Liberalism ( pdf http://www.adlaitoday.org/ideas/archive/care1_liberalism_08-28-52.pdf) (28 August 1952)
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Adlai Stevenson 131
mid-20th-century Governor of Illinois and Ambassador to the… 1900–1965Related quotes

Source: Secrets of the Millionaire Mind: Mastering the Inner Game of Wealth

“There are a great many doors open; but a door must be of a man's size or it is not meant for him.”
As quoted in Henry Ward Beecher: His Life and Work (1887) by J.T. Lloyd, p. 261 http://books.google.com/books?id=MV5DAAAAYAAJ
Other Sourced

“You can tell the greatness of a man by what makes him angry”

“Anyone who tells you size doesn't matter has been seeing too many small knives.”
Source: Narcissus in Chains

“Size is not grandeur, and territory does not make a nation.”
"Address on University Education" (1876) http://aleph0.clarku.edu/huxley/CE3/Ad-U-Ed.html, delivered at the formal opening of Johns Hopkins University, Baltimore, Maryland, September 12, 1876. Huxley, American Addresses (1877), p. 125. Vice President Hubert H. Humphrey used the same words in a commencement address at the Holton-Arms School, Bethesda, Maryland, June 1967; reported in The Washington Post (June 11, 1967), p. K3
1870s
Context: I cannot say that I am in the slightest degree impressed by your bigness, or your material resources, as such. Size is not grandeur, and territory does not make a nation. The great issue, about which hangs true sublimity, and the terror of overhanging fate, is what are you going to do with all these things?

“It's not the size of the dog in the fight; it's the size of the fight in the dog.”
Anonymous American proverb; since 1998 this has often been attributed to Mark Twain on the internet, but no contemporary evidence of him ever using it has been located.
Variants:
It is not the size of the dog in the fight that counts, but the fight in the dog that matters.
"Stub Ends of Thoughts" by Arthur G. Lewis, a collection of sayings, in Book of the Royal Blue Vol. 14, No. 7 (April 1911), cited as the earliest known occurrence in The Dictionary of Modern Proverbs, edited by Charles Clay Doyle, Wolfgang Mieder, and Fred R. Shapiro, p. 232
It is not the size of the dog in the fight that counts, but the fight in the dog that wins.
Anonymous quote in the evening edition of the East Oregonian (20 April 1911)
What counts is not necessarily the size of the dog in the fight — it's the size of the fight in the dog.
Dwight D. Eisenhower, declaring his particular variant on the proverbial assertion in Remarks at Republican National Committee Breakfast (31 January 1958) http://www.presidency.ucsb.edu/ws/index.php?pid=11229
Misattributed

“A drowning man does not complain about the size of a life preserver. ”