The smallest of the true gars is the Shortnose Gar (Lepisosteus Plastomus) topping out at 33" in the wild. They can be housed for a pretty long time in a 24" deep tank if you get a juvenile, though they will outgrow it over time.
Although it isn't a true gar, you could get a Needlefish or Needlenosed gar. They aren't related to gars, but they are pretty cool and only get a foot long.
I'm starting on my tank today and was wondering the same question about a garfish.I'm putting a largemouth in the tank and was thinking about a gar as well.