Is there a critical mass to the length of the panes?

Here's a question. My greenhouse lacks glass, all of the 12'x10' of it but I have been lucky enough to acquire a large supply of Dutch Lights. If I cut them down to 2 foot widths I can re-glaze the walls in single panes but not the roof. The roof panes would be over 1800mm long but the Dutch Light only 1422 meaning if I were to use a full length and a short bit I would have a long pane supported at one end and both sides only. So to the question, is there a critical mass to the length of the panes beyond which the glass might fail, have I got to cut the lengths down and have two or three panes? Any advice gratefully received.