If you're looking for direct match to the OEM 2016-2018 Civic spare (which fits perfectly into the Insight's tire storage space since same body dimensions as Civic), the tire dimensions are T125/80D16 97M.
Even after adding the OEM "foam tool holder" for
either the GenX Civic (part #84545-TBA-A10 - round shape) or the Gen3 Insight (part #84541-TXM-A10 - horseshoe shaped) fits perfectly within the space atop the compact spare, keeping the floorboard flush. Pictures of the fit of each of these can be found in
this post within the thread @hasarard referenced.