If Adventure Creator, you can adjust the Text size slider in the MenuInventoryBox element's properties. Otherwise, you can modify the Text component's Font Size value.
Just to let others know, my first solution was to change the second parameter of the SetFont function in MenuInventoryBox.cs, 50 was a reasonable enough size for me. The suggested solution didn't work as my inventory boxes didn't have any text component. But I think it should work if you add a child with the text component.
Comments
If Adventure Creator, you can adjust the Text size slider in the MenuInventoryBox element's properties. Otherwise, you can modify the Text component's Font Size value.