Are you just talking about how long the tone is sustained when pressing a key, or the real sample length? These are two different things, just to make sure there's no misunderstanding. There can be a short sample which is looped, so the tone can still have a very long sustain of 10 seconds or more. In a synth you could edit the piano tone to endless duration by changing the amplitude envelope even if the sample of the real piano tone is short, I guess you know that? (Except probably the Krome/Kronos, as it has unlooped piano samples, which decay as the real piano does, so probably you can't edit it to endless, I'm not sure about that.) I don't know if these things can be changed in a PSR arranger.

Here's a nice graph of the envelope I'm talking about:
http://www.nathansheldon.com/xp-80/TVA_envelope.html