Actually I think its a special size. if you were fitting your own system you would need an adapter panel that replaces the whole existing radio. However not sure if they are avaliable yet for the Punto Evo. Had a quick look on Ebay and couldn't see anything.
"These adaptors will convert the non din-sized aperture left after removing the factory stereo to a Single / Double din size ready for head unit installation."
I know fiat (*atleast used to) make one to fit the Grande punto so might be worth checking with your dealer it might be a better colour match than most of the aftermarket ones seem to be.....
"These adaptors will convert the non din-sized aperture left after removing the factory stereo to a Single / Double din size ready for head unit installation."