1/2 size won't do without amplification. You need 3/4, which is actually the standard.
If you want a low, small acoustic bass instrument, try a cello. Doesn't go quite as low as a bass, but it can serve the bass function and will be loud enough if played with a bow, maybe even without the bow.
Both double bass and cello are instruments requiring large amounts of practice to master, but if you want to play 3-chord tunes in keys with open strings, and that is it, you can get enough technique together easily.
You don't need a half-size, and I do not see how you can recommend a cello over a half-size when the half-size has a much larger body. One of these will do:
It's a 1/4 size DB, and I know from experience that it can fill a falrly large coffee house even when the two guitars and two vocalists are miked and it is not.