Question Small spare wheel


Hello, I am the new owner of an A3 Sportback 2005 2.0 TDI 140 HP, after my euphoria passed I looked to see what the spare wheel had and the surprise when I lifted the mat from the trunk, there is a locusr there but it does not fit under either it forms a normal-sized wheel, which doesn't really suit me, but I guess there's nothing more that can be done other than to settle for a reduced-sized wheel that will have to be ordered, how does such a reduced-sized wheel behave, does it have certain restrictions in operation? Does anyone know approximately how much such a wheel costs? Thanks.