http://images.google.com/images?q=ot...=1&sa=N&tab=wi
A quick search on google will show you load of pictures for otocinclus. Most of them grow up to 5 cm on the average.
Here's one from AQ's gallery taken by V31.
Those from Nature Aquarium will serve you fine. Usually they have Otocinclus affinis or Otocinclus vittatus.
Cheers,
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