To change the size the squares, add this to the end of the URL:

&square_size=d

where d is the size of each square in pixels. The default is 35 pixels per square. Here’s a smaller board with 25 pixels per square:

http://www.eddins.net/steve/chess/ChessImager/ChessImager.php?
fen=rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR/&square_size=25