If you don't want something "as large as a Prius", as you say, then I offer the advice of doing what I did recently ... Buy a used Honda Insight.  I was able to get a 2005 with 37k miles on it for less than $15k.  This was an extraordinary deal, I admit, but you should be able to find one for better than $18k, especially if you are willing to go out of state to pick it up.

 

I love the car, it is currently getting me 56mpg on premium gas on my 65 mile roundtrip daily commutes to/from work.  This is about 80% highway miles, and I am rarely under 65MPH when traffic conditions permit.

 

The Insight is a 2-seater, with no trunk, so it is strictly for commuting, or runs to the store.  My golf clubs and handcart will fit, but just barely...

 

Good luck!