In timed mode, achieve a score of 50 points or more. Though no 
notification will be given, a new button labeled "??? OFF" will
appear on the options menu. Press it to toggle it like SFX and 
music. Afterward, in both game modes, the grapes' sizes will 
vary from approximately 1/2 to 3 times the ordinary grape size.
In addition, the title screen will display the diameter in inches
of the largest grape you have caught and kept on the plate until 
the end of a game. 

Unlockable            How to unlock
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Variable Grape Size - Achieve a score of 50 points or more in timed mode.