“Absolute size by itself is no indicator of success and achievement, let alone of managerial competence. Being the right size is.”
Source: 1960s - 1980s, MANAGEMENT: Tasks, Responsibilities, Practices (1973), Part 3, p. 672
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Peter F. Drucker 180
American business consultant 1909–2005Related quotes

“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

“I have an ego the size of a small planet, but I'm not _always_ right […].”
Re: clarification on git, central repositories and commit access lists, 20 Aug 2007, Linux Weekly News, kde-core-devel@kde.org, 2007-12-28 http://lwn.net/Articles/246381/,
2000s, 2007

Remarks at Republican National Committee Breakfast (31 January 1958) http://www.presidency.ucsb.edu/ws/index.php?pid=11229; Eisenhower hear delivers his particular variation of a pre-existing proverb, which has since become widely dispersed as simply "It's not the size of the dog in the fight; it's the size of the fight in the dog." In that form it has become widely attributed to Mark Twain on the internet, as early as 1998, but no contemporary evidence of Twain ever using it has been located. The earliest known variants of it occur in 1911, one in a collection of sayings "Stub Ends of Thoughts" by Arthur G. Lewis, in Book of the Royal Blue Vol. 14, No. 7 (April 1911): "It is not the size of the dog in the fight that counts, but the fight in the dog that matters", as cited in The Dictionary of Modern Proverbs, edited by Charles Clay Doyle, Wolfgang Mieder, and Fred R. Shapiro, p. 232, and the other as "It is not the size of the dog in the fight that counts, but the fight in the dog that wins" in the evening edition of the East Oregonian (20 April 1911) http://listserv.linguistlist.org/pipermail/ads-l/2015-October/139250.html
1950s

“For I am the size of what I see
not my height's size.”
Attributed to the Caeiro alter ego, in A Factless Autobiography, Richard Zenith Edition, Lisbon, 2006, p. 71
The Book of Disquiet
Original: Porque eu sou do tamanho do que vejo
e não do tamanho da minha altura.

“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)

volume I; lecture 1, "Atoms in Motion"; section 1-2, "Matter is made of atoms"; p. 1-3
The Feynman Lectures on Physics (1964)