From the Super Mario Wiki
The Mini-Mega Hammer is an item found in Mario Party 4. The hammer either shrinks or enlarges the player who is hit on the head with it, giving them the effect of a Mini Mushroom or a Mega Mushroom. The effect is random. The player holding the hammer cannot use the item on themselves. The player should be wary when using this item on someone else near them, because if the hammer makes the player mega, the target can easily steal coins or use the extra dice to move further. However, it can be used to make another player bypass the Star Space and other such events. If the target is changed to mini, they can possibly fit through the many gates of the maps. This could allow the player to play several Mini Mini-Games. However, their Dice Block will still only go from 1 to 5.
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