Here's my 18" spare with 145/80/R18 tyre. Fits in the well with room to spare and sub still fits in using the standard bolt. All fits under the floor flush, so still plenty of room for tools, shopping bags, luggage straps and nets etc under the floor boards.
Can you remember which Ebay shop you bought the 18" spare wheel from?
Probably won't help you, but I thought someone on here had a 19" and it fitted over the speaker, might be wrong though.
2016 Black 1.2 TCE Signature Nav S Manual Petrol Bose with Tech Pack and Drop Tow Bar
Running Rlink 2 Version: 220.127.116.111 Boot 5661
Self activated Android Auto
Map Version: mappackage_tt_feu_mid_2018_06