I was looking at the specifications section in the online Owner's Manual and came across the spec for the Compact Spare, which is for "Vehicles with optional spare tire" (p.653). In fact, there is a wheel-well and a bracket to take a screw-down clamp for such a tire under all that Styrofoam in...
Thanks for your reply. I don't worry about a flat tire yet, but I need a spare to do a tire rotation because I don't have two jacks. I guess now I can look into eBay and see if I can get one.
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.