There were smaller sizes than many realize during this period. The sizes went as follows: whole plate: 6 1/2 x 8 1/2 / Half plate: 4 1/4 x 5 1/2 / quarter plate: 3 1/4 x 4 1/4 / sixth plate: 2 3/4 x 3 1/4 / eighth plate: 2 1/8 x 3 1/4 / ninth plate: 2 x 2 1/2. Anything larger than 11x14 was considered mammoth plate but most typically it was 18x22 or 20x24.
To follow on with this - the vast majority of photos of that period were done on quarter-plate or smaller. I've got a small but growing collection of antique images - I have ONE half-plate tintype, ONE quarter-plate Daguerreotype, and the vast majority of the rest are 1/6 plate images. Another medium that was popular in that day and age was the carte-de-visite - an albumen print pasted on to a piece of card stock with the photographer's information printed on it, and rarely identification of the subject. Most cartes-de-visite are about 1 1/2" x 2 1/2" or thereabouts. This was the normal size image from Brady's studio (one of the cdv's I have is a Brady, from his New York operation). Some of the illusion of size we have of these images comes from seeing the images reproduced in modern books and on tv.