A ~10cm African Butterfly Fish, providing there are no small torpedo-shaped fish as tankmates. Something very sedatory is your only responsible way of going relatively big in such a small tank.
That's a very difficult one as large fish generally need large tanks. Can you give us the tank dimensions and water parameters? Because the length, width and height all affect which fish would be comfortable in a tank that size, and it would be quite dangerous to suggest hard water fish for a soft water environment.
Assuming the tank is a 60 cm long one, then probably a Betta (2+" including tail) or a pitbull pleco (2") are the largest fish that it will hold. As a general rule of thumb, a 60 cm tank limits one to up-to-1" mid-water schooling or active fish and up to 2" for passive fish.
If you like, have a look at this list of fish that I have put together which are suitable for a 60*30*30 cm tank: http/blog.natureaquarium.co.uk/?page_id=278