Of the species commonly available, the only spearer is Pseudosquilla ciliata. They cruise around a lot and dig, so a 12 gal nanno might work, but you would not want one bigger that about 3 inches. The species gets to almost 4", but is very slow growing.
Note of the Haptosquilla or Gonodactylus grow that fast either. A two inch animal will grow a maximum of another inch in a year, probably less, so I would not worrry about the animal outgrowing the aquarium.
Roy