Kertesz is one of my favourites.
His earliest work is in contact prints. These are reproduced in their original size in the most recent Kertesz book (Andre Kertesz by Sarah Greenough, Robert Gurbo, and Sarah Kennel (Hardcover - Jan 24, 2005)) and have a beauty of their own.
In another book (In Focus: Andre Kertesz: Photographs from the J. Paul Getty Museum (In Focus) by Andre Kertesz (Paperback - Sep 29, 1994)), he is said to be a master darkroom craftsman, and indeed when he emigrated to Paris his declared profession was "darkroom technician. In the same source, the long side of many pictures are a few milimeters shy of 25cm. So I guess 20x25cm paper was a kind of standard. But larger prints were not uncommon.
He frequently cropped his pictures, often drastically. The most striking picture is "Me and Elisabeth", with his arm on the shoulder of Elisabeth. In the earlier years the print is full frame, whereas re-prints towards the end of his life show a severe crop, only showing Elisabeth's face and his hand on her shoulder.
As commented above, the printer Igor Bakht took over printing Kertesz's work during his later years. In Celebrating the Negative by John Loengard (Hardcover - Oct 26, 1994), Bakht recalls that Kertesz disliked high contrast prints. When Bakht emphasized the clouds in a print too much, Kertesz asked for them to be less dramatic.
I've been to a Kertesz exhibition in Istanbul last year and the original prints are truly exceptional. The print quality is unfortunately lost in all the Kertesz books I have.