My opinion.... and from what I've known people to do..... its whatever size you can wedge into the car. Unless you are trying yur best to return it to complete stock.
The dask speakers I guess are the hardest... but I think they are like 2 or 2.5 inch. As far as the back... I'd try to at least put 6x9. If you plan on subs in the trunk though... you'll either need baffles or enclosures for the 6x9s. Yes, you can put and enclosure around them while mounted in the stock position. And a baffle is just a rubber boot that sourrounds them. If you dont use these, the subs will have a habit of bottoming out the 6x9s.