Many of the tropical fish seen in pet stores are not fully grown.  The final size of the fish varies dramatically between species.  This is a list of the sizes that individual species are likely to grow to.  Only some species are listed here, and adult fish of the same species vary somewhat in size.  This is meant as a general guide to help you avoid buying fishes that will outgrow your tank.
Oscars: Astronotus ocellatus
Tinfoil Barb: Puntius schwananfeldi
Bala Shark: Balantiocheilus melanopterus
Apollo Shark: Luciosoma spilopleura
Six-barred Distichodus: Distichodus sexfasciatus (may grow larger in wild)
Banded Leporinus: Leporinus fasciatus
Red Piranha: Natterer’s Piranha: Rooseveltiella nattereri
Clown Loach: Botia macracantha (may grow larger in nature)
Green Snakehead: Channa gachua, Ophiocephalus kelaartii (often only 8 in)
Four-eyed Fish: Anableps anableps
Cuckoo catfish (female): Synodontis multipunctatus
Royal Panaque, Royal plecostomus: Panaque nigrolineatus (16 in)
Gold Nugget Plecostamus: Scobinancistrus sp.
Giant Whiptail, Golden Whiptail: Sturisoma aureum

Sources:

Butler, Rhett A. 1999-2008. Mongabay.com – San Francisco, CA

Practical fish keeping http://www.practicalfishkeeping.co.uk/pfk/pages/home.php

Mills, D. 1987 Illustrated Guide to Aquarium Fishes. Galaxy Press, England