Don't assume that just because something works one way in iOS, that's how it works in Android. As Vineet alludes to, in Android's C2DM only deals with data. It does not deal with UI. Adding a notification in the notifications tray is a common thing to do with that push notification, but it's not directly related. As noted in other answers, the message size limit is 1024 bytes.
If you're concerned about how many characters can fit in a notification in the tray however, there is technically no hard limit. For one, Android does not use fixed width fonts (by default) in the tray. A string of 10 characters like "llllllllll" will be a different width than a string of 10 characters like "WWWWWWWWWW". Additionally, you can do custom views in notifications, meaning you can change the font size which would change the number of characters that you can fit.