Your sponsor is a Canadian PR, so a birth certificate from her home country would be rather redundant, which is why you sent a copy of her PR card instead.
My sponsor was born in Ontario, which is why I asked about the birth certificates. :)
Are you sure that a birth certificate is necessary at all? My wife was approved as sponsor, and we didn't send any birth certificate in with the application. We sent a copy of her PR card, and we may have sent a copy of her passport.
The pocket-size is fine. Or you can submit a copy of his/her passport. Follow the country-specific guidelines for the country that you are applying from.
Which country are you applying from? You'll have to check the country-specific guidelines to see if another document would be acceptable in place of a birth certificate.