“I have never known a worthwhile man who became too big for his boots or his Bible” is frequently credited to Abraham Lincoln (1809-1865), but there’s no evidence that he ever said it. The saying “too big for one’s boots” means someone who believes he or she is more important than he or she actually is.
 
The earliest citation, from 1910, credits an English woman:
 
“An English woman of keen observation says: ‘I have never yet come across a person really far above the average, either mentally or morally, who ever became too big for his boots or his Bible.’”
   
   
Wiktionary: too big for one’s boots
Adjective
too big for one’s boots
1.(idiomatic) Far less important or capable than one’s thinks or claims to be.
Synonyms
. too big for one’s britches
     
Google Books
A dictionary of slang, jargon & cant embracing English, American, and Anglo-Indian slang, pidgin English, gypsies’ jargon and other irregular phraseology
By Albert Barrère and Charles Godfrey Leland
London: G. Bell
1897
Pg. 350:
Too big for his boots (theatrical), a phrase invented by the late FB Chatterton, manager of Drury Lane, to denote an actor who, having made a hit, gave himself airs, and became obstreperous and presuming.
   
Google Books
How to Develop Self-Confidence in Speech & Manner
By Grenville Kleiser
New York, NY: Funk & Wagnalls Company
1910
Pg. 227:
An English woman of keen observation says: “I have never yet come across a person really far above the average, either mentally or morally, who ever became too big for his boots or his Bible.”
 
Google Books
An American Speaks His Mind
By Henry J. Taylor
New York, NY: Doubleday
1957
Pg. 230:
In my own experience and travels, at least, I’ve never seen or come across a person really far above the average either mentally or morally who ever became too big for his boots or his Bible.
 
9 May 1975, El Paso (TX) Herald-Post, “Mother’s Day” by Henry J. Taylor, pg. B2, col. 4:
In my own experience and travels, I have never seen a really superior man who ever became too big for his mother, his boots or his Bible.
 
Google Books
They Never Said It:
A book of fake quotes, misquotes, and misleading attributions
By Paul F. Boller and John H. George
New York, NY: Oxford University Press
1989
Pg. 82:
BOOTS-AND-BIBLE QUOTE “I have never known a worthwhile man who became too big for his boots or his Bible.”
 
Lincoln was said to have made this remark during the darkest days of the Civil War, but though he read and reread the Bible, and was deeply attached to it, there is no good evidence that he ever said this about it.