I've got a full size spare but if I give up my OEM wheels I will probably look at selling all 5 and getting a space saver spare for the boot. Two questions come to mind:
1 are all compact/space savers the same size?
2 do you turn traction control off when you use one?

 

Mohrd, I personally would not recommend using a space saving spare tire. Keep in mind that you have AWD in this car. All 4 tires need to be spinning at the same speed at all times or you can get excessive binding in the drivetrain leading to damage to parts. This isn't too much of a problem with a 2wd vehicle as they tend to have open differentials and having 1 tire spin faster than the other isn't a big deal. But, with our cars, it isn't that simple.

I would hate to see you hurt the car trying to save some weight/space.

As for the spare tires, no, not all are the same size. They are essentially a light build rubber tire (ie, very little rubber put into the tire, hence why they are limited in speed and weight handling ability). If you got a compact spare that was the same overall diameter but narrower, then you would be fine (all the wheels would spin at the same rate). But, trying to find a tire like that is going to be very hard.

Personally, you would be better off pulling the spare tire completely and then keeping a can of fix-a-flat in your trunk. The only thing I will caution you on this stuff is once you use it, the next time you need to add air to the tire, you will end up having to unmount the tire and buy a new one due to gumming up of the valve stem and the gunk that will be left inside the tire. Also, once you add the fix a flat, make sure to drive the car or you will have a tire that is severely out of balance. I've seen people do it hence why I point this what would appear to be "common sense" things.

 

I kinda meant a Jaguar factory supplied space saver...like, are all the compact spares that came with the X-type the same one or did they differ by years or wheel size that the car came equipped with? Whether our Xs came with 16, 17 or 18 inch wheels, the diameter is all right around 25 inches give or take with the different tirer profiles so I would assume there was no real difference. If I end up parting with my 17 inch Sport wheels I was going to throw in the 5th.

 

mohrd, atleast with my car, I have a full size spare. I don't think Jag put the space saver tires with the X-Types for the reason I stated above. maybe with the other models that are not AWD, but the X-Types I believe all got full size spares.

All I can say is to look around. You have the right idea in staying with the same diameter. See what you can find and go from there. Worst case, you can get a basic steel rim with the correct bolt pattern and then find a tire that is really close in size that would meet what you are after. Not like you will be doing over 70 mph when on the spare. So, any speed rated tire can be used, making it easier to find one that would work.