Anita Roddick (1942-2007), founder of The Body Shop, had a saying on her company’s trucks in 1993:
 
“If you think you’re too small to have an impact, try going to bed with a mosquito.”
 
“If you think you’re too small to make a difference, try going to bed with a mosquito in the room” was said (in a book published in 1998) to be an African proverb. The origin of the mosquito proverb is unknown, but Roddick is most frequently cited for having said it.
 
     
25 January 1993, Time magazine, “Anita Roddick: Anita The Agitator” by ‎Philip Elmer-DeWitt:
Roddick’s current favorite, taken from the side of one of her company’s lorries: IF YOU THINK YOU’RE TOO SMALL TO HAVE AN IMPACT, TRY GOING TO BED WITH A MOSQUITO.
(Reprinted in the Lakeland Ledger, February 19, 1993—ed.)
 
Google News Archive
17 November 1995, Bryan (OH) Times, Entertainment, pg. 16, col. 1 ad:
IF YOU THINK 25 CENTS IS TOO SMALL TO MAKE A DIFFERENCE, TRY TAKING A NAP WITH A MOSQUITO.
(Artesian Water Conditioning—ed.)
     
Google Books
Servanthood:
Leadership for the Third Millennium
By Bennett J. Sims
Cambridge, MA: Cowley Publications
1997
Pg. 34:
Anita Roddick, founder of The Body Shop, paints a slogan on her trucks: “If You Ever Feel Too Small to Make a Difference, Try Getting in Bed with a Mosquito.”
 
Google Books
Awakening:
The Upside of Y2K
By Judy Laddon, Tom Atlee and Larry Shook
Spokane, WA: Printed Word
1998
Pg. 37:
And as an African proverb reminds us, if you think you’re too small to make a difference, try going to bed with a mosquito in the room.
 
18 December 2002, Ithaca (NY) Journal, “Philanthropy is all about taking action” by Jerry Dietz, pg. A10:
Those who think something small can’t make an impact have never been bitten by a mosquito.
 
Google Books
The Secrets Of Happiness:
100 Ways to True Fulfilment
By Ben Renshaw
London : Vermilion
2003
Pg. 95:
The Body Shop once had a slogan that read, ‘If you think you’re too small to make a difference, read up on mosquitoes.’
 
Google Books
Nothing’s Too Small to Make a Difference:
Simple Things You Can Do to Change Your Life & the World Around You
By Wanda Urbanska and Frank Levering
Winston-Salem, NC: J.F. Blair
2004
Pg. 55:
“If you think your actions are too small to make a difference, you’ve never been in bed with a mosquito.” — Anonymous
   
Village Voice (New York, NY)
NY Mirror
By Rob Brezsny
Tuesday, Jun 21 2005
(...)
VIRGO(Aug. 23–Sept. 22): What I’m about to tell you is always important to keep in mind, but it’s especially crucial right now. If you think you’re too small and insignificant to have a major impact, you’ve never spent the night in bed with a mosquito.