Long time lurker... just went through a similar process to the people above, thought to clarify a few points that might be useful to anyone with the same questions/issues
The Mk3 Superb comes with 3 official options for "insuring mobility" or however Skoda marketing calls them
Tire repair kit with compressor and goo - under the floor there is a moulded polystyrene piece with extra storage space - like in @newbie69's picture. It can be easily removed by pulling out the floor cover (lift and pull towards you) and then pushing the moulded piece up and away from you (towards the backseat). The floor is perfectly flush and level when it's installed but you are operating on the assumption that any issues you might face will be small and fixable with the goo... 4-6 mm max damage size, not too close to the tread edge and that the rim is still in good shape - good for fuel economy and some bonus storage space, bad for peace of mind
Standard spare wheel - it's a 205/55 R16 (a plain-jane 91 load-index asymmetrical summer tire) mounted to a 16x6.5 ET41 VW-spec ( 5x112 CB 57.1) steel rim. It comes with the small-diameter tool kit moulding that you can see in the pictures above - if you want just the kit, the Skoda part no. is 3V0 093 860 B - should cost about 60-ish EUR. The kit INCLUDES the floor attachment/fastener so you have all you need in the package. The wheel is in one of the most frequent sizes in Europe so feel free to avoid the dealer markup and buy it from a tire shop or elsewhere online. The tool kit and the upper side of the wheel protrude a little above the horizontal, so you have a 2-3 mm tall "hump" in your load area that slowly becomes visible with usage. The wheel is SIGNIFICANTLY smaller than the standard fitment so it is a temporary fit and you are limited to 50 mph/80 kph. Because of the difference in diameter (671mm for the standard 19in vs 632mm for the 205/55R16) many functions will be unavailable (ACC and the like) and there will be some complaining from the car. For slightly better results, the spare should be fitted - if you've got the patience for it - to the rear (undriven) wheels. Most impacted are the people with a 4x4 transmission because the Haldex diff really doesn't like different tire sizes - so for its sake, please fix the proper wheel quickly and put it back on. This size is meant only for the <200HP cars (actually that would be all but the higher-powered 2.0 TSIs ) because it can't clear the front brake calipers on those - it will clear the rear ones though - so in theory you could still use this on the bigger TSIs but with the caveat that if you blow a front tire you need to mount the spare to the rear and then use the rear tire to replace the blown front tire.
Narrow spare wheel - 125/70 R18 comes as standard/option - depending on the market - on the higher-powered 2.0 TSI versions of the Superb and can clear the front brakes on those. It comes with the larger-diameter moulding (the one that looks like it has petals) that you can see in @Ivan8192 's photo above. The petals are meant to fill the space above the narrow wheel ( just 125 mm wide) and support the load-bearing floor so that it doesn't bend/collapse with a heavier load. The wheel is pretty standard for the VAG group and Skoda sells it for a bit under 200 EUR - with the toolkit being another 60-ish on top - the toolkit includes the floor attachment plastic screw thing. The code for the wheel is 3V0 093 860 B and the one for the toolkit (if you only want that one) is 3V0 093 860 C - there is no point though in getting that kit for a non-narrow wheel . The wheel is smaller-diameter than the standard wheels, same as the one above (632 mm) so the same problems apply (no Cruise-control, not so great for the Haldex, etc) - and with it being so narrow, I would feel really uncomfortable with it as a front wheel at high speed - so the same idea with mounting the spare only on the rear and then putting a full-size rear tire on the front applies
Unofficial options
Full-size wheel - 215/60R16 or 215/55R17 or 235/45R18 or 235/40R19... the plastic trim at the back of the trunk must be partially cut to allow the wheel to fit - what @JR RS did and the load floor will be raised because of the wheel width (by 12-15 mm for the 215 wheels or by 30ish mm for the 235s ). You can then either raise the floor to keep it level - what @Tokebluff did or just ignore it if it's hidden under the variable load floor... OR...
Alternate-size wheel - i went a bit weird for this one - 205/65R16 - it is the same diameter as the standard wheel (673mm vs 671mm for the 19in) so there should be no complaints from the car's systems (ACC works, Haldex is happy and all that) and with the 205 width it fits just as the 205/55R16 would under the trunk floor. The only issue is that it must be inserted from the the front back rather that dropped straight down - the side towards the bumper must go in first to clear the trim there. It DOESN'T clear the front brakes on the higher-powered 2.0 TSIs but it can be installed on the rear (same story as above) and the rear tire used on the front - and then you can drive with full speed. Typical install is on a 16x6.5 ET41 5x112 CB57.1 rim with a very slight (~1mm) lift of the load floor - BUT - if you want to have it absolutely flush, you can mount it to a slightly narrower rim (16x6 rim, ET anywhere between 33 and 48, same specs otherwise)