It's not so much about sensor size as dpi (dots per inch )of the image you produce. You will need 300dpi for a magazine. THe confusion is, you can produce a 300 dpi image from a camera phone, in Photoshop, only it will look terrible. Personally, I'd go for anything over 5 or 6 megapixels, anything under and you'd struggle. That said, I used to be able to print A4s off a 3 megapixel camera, but, and it's a big but, it had a great lens.
Pixelcount is a bit of a myth in this respect, as you could have a 15 megapixel camera with a terrible lens, and get big images, but which have no clarity. For me, over about 5 megapixels, and then the lens starts becoming the important thing.