A sig's size and shape isn't limited to a rectangle, can be anything to fit the look that you have in mind. Cropping usually helps to focus the eyes on a particular part and c4d is an option but it isn't necessary. Again, it's all up to what sort of look you want to go with it.
Because your characters are rather cute, c4ds may not exactly fit. Also because they are rather stationery... you can use vector brushes to add movement to the sig.
Important thing to remember with most designs is the rule of thirds. Put the focal points into the intersections (that are created when you divide the plane in thirds horizontally and vertically).
EDIT: I have been using this:
http://senthrax.deviantart.com/art/F...orial-55042010
The top half is a must read. Lots of great info on design.
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