Banner Size is a tweak released during this night in Cydia, and with its help you can change the size of iOS notification banners. The tweak has a settings menu in the Settings app of iOS, and there you can choose one of over 15 sizes specially configured for them. The tweak also allows you to change the positioning of the banner, so that it can be displayed on the left/right side of the screen, or even in the central part, all depending on the size and model chosen.
Banner Size is available for free in Cydia's BigBoss repo.
