I'm resurrecting this thread as I don't think the 'definitive' answer ever appeared all on one post, and to help anyone who reads it all again...
For the non 4x4 and non Trekking Panda models, when supplied with a spare wheel, there is a plastic insert inside the spare which holds the jack and other tools. That space saver spare wheel is 14" in diameter and the jack is quite compact.
For the Trekking, 4x4 and Cross, the spare wheel needs to be 15" in diameter (in part to clear the larger front disc brakes), and is fitted with a 125/80x15 space saver tyre. And, crucially, the jack is different - because the car sits 2" higher of the road to start with, it uses a bigger jack that extends to a greater height. This jack is labelled as being for the Panda 4x4 and Qubo models only. This will not fit inside the spare wheel and so is supplied in a separate bag. The 175/65 R15 wheels that the Trekking/4x4 are fitted with, and the 185/65 15 wheels of the Cross will not fit into the spare wheel well in the boot, so you cannot carry a full size spare.
Note also that the towing eye is different for the Trekking/4x4/Cross. It is longer, to reach through the deeper bumpers these cars have. The shorter towing eye, supplied with other Pandas is too short to reach fully into the threaded mounting on the car.