“'The mind, once stretched by a new idea, never returns to its original dimensions.' That's from Emerson, by the way. All I was hoping to do is stretch some goddamn minds.”

Source: Think Big (1996), p. 224

Also reported as "One's mind" instead of "A man's mind", and "can never go back" or "never regains" instead of "never goes back"; most likely properly attributed to Oliver Wendell Holmes, Sr.
Misattributed

Often given as: A mind that is stretched by a new experience can never go back to its old dimensions.
Or: A mind that is stretched by a new idea can never go back to its old dimensions.
Actually by Oliver Wendell Holmes, Senior, from " Autocrat of the Breakfast Table https://books.google.com/books?id=BoQ3AQAAMAAJ&pg=PA502&dq=%22stretched+by+a+new+idea%22&hl=en&sa=X&ved=0ahUKEwidspn60tTJAhVJ1GMKHbt3Bn0Q6AEIHTAA#v=onepage&q=%22stretched%20by%20a%20new%20idea%22&f=false", originally published in The Atlantic, September 1858.
Misattributed

The Autocrat of the Breakfast Table (1858)

Actually said by Oliver Wendell Holmes, Sr. in his book The Autocrat of the Breakfast Table: "Every now and then a man's mind is stretched by a new idea or sensation, and never shrinks back to its former dimensions."
Misattributed

Source: The Particular Sadness of Lemon Cake

25 November 2011, Twitter
Speaking & Features

Dita Amory, in Pierre Bonnard: The Late Still Lifes and Interiors; Yale University Press, New Haven, 2009 - ISBN 978-0-300-14889-3, p. 4
Bonnard started to paint usually on an unstretched canvas

Source: Mama Day