Actually, the better the source material the bigger the print you can make. Pixels are one thing, but if you're blowing up noise you won't be happy with the results. I've known people with a
Nikon D2H (a 4MP camera) who have successfully made
20x30" prints. This takes a very good sensor (camera phones are just now getting to "good") and an excellent photographer (and if you're looking to blow up a camera phone print to 20x30 you probably don't qualify). Don't expect this with a camera phone, but 150dpi is still very good quality and gets you into the 8x10 range. That said, no matter how many megapixels you've got, camera phone images rarely look great at more than 4x6" or on a ~2MP 1080p TV screen/monitor without retouching. In essence, if you can tell the difference between 150dpi and 300dpi you probably don't want the image at that size anyway.