One option for a spare tire is a compact temporary tire that is small in size. It's much slimmer than your average tire. While it can replace a flat tire, it can't be driven for long. Get your tire repaired or replaced immediately.
A full-size spare tire is something that can be driven on for the rest of its lifetime. Just make sure it's filled with the appropriate amount of air if it has been sitting in your trunk for months or years.