There are two sizes of ads that appear in stations. Usually, they are about 30" wide by 60" high, but some ads are double width. In most cases, stations have two 60" wide recessed panels for mounting advertisments, and they in turn are grouped in pairs between station name signs. At one or two points on the platform, one half-width panel will contain a subway map. In stations that have been refurbished, this panel is usually mounted on the new tilework with a metal frame.
The thumbnails below link to photographs of various ads found in the system in the recent past. They are provided for the purpose of modelers desiring authentic ads for their model subway stations. The images can be downloaded by right-clicking on the thumbnail and selecting "Save Target As", specifying a filename to save it under, as well as it's location on your hard drive. You will need some sort of image-editing program to print the photos at something resembling 'O' scale . Size reduction should be performed with the printer itself (i.e. on a print preview page where the output can be expressed in actual dimensions or size relative to the sheet of paper), rather than shrinking the image on-screen, since your printer is capable of much finer resolution than your monitor. Half-size ads scale out roughly to 5/8 of an inch wide and 1 1/2 inches tall. Double-size ads would be about 1 1/2 inches square, though they may actually turn out slightly wider than they are tall. Needless to say, much of the detail will disappear at such a small size, but they'll look reasonably authentic from a normal viewing distance. More ads will be added in the near future for variety, but depending on the length of your station platforms, some ads may appear more than once, just like the prototype.