var locked;
editor.on( 'key', function( evt ){

var currentLength = editor.getData().length,
maximumLength = 200;
if( currentLength >= maximumLength )
{
if ( !locked )
{
// Record the last legal content.
editor.fire( 'saveSnapshot' ), locked = 1;
// Cancel the keystroke.
evt.cancel();
}
else
// Check after this key has effected.
setTimeout( function()
{
// Rollback the illegal one.
if( editor.getData().length > maximumLength )
editor.execCommand( 'undo' );
else
locked = 0;
}, 0 );
}
} );