The famous 9 foot model 'D' if I'm not mistaken.
An interesting comparison on interpretations:
Rachmaninov:
YouTube - Rachmaninov Plays Rachmaninov - Prelude in G Minor
Horowitz:
YouTube - vladimir horowitz plays rachmaninoff prelude op.23 n.5
Gavrilov:
YouTube - Gawrilow Rachmaninoff Prelude g minor (op 23,5)
:beer2: