Depends what you mean as standard and what film you have. For 35mm film the standard size is 5x3.5inch, then they brought in 30% bigger, which is 6x4inch, then 100% bigger which is 7x5 inch.
.
I'd say 5x3.5 is standard size as they're what the others base their % bigger claim on.
I think you'll find it all depends on the the film used as the photograph is in fact the image on the film and not the piece of paper others are talking about as that is the print.
I work in the scanning/graphics department of a local newspaper, and I can say with some confidence that there is no standard size for photographs. They come in sizes of two inches wide to over twenty, and even bigger.