This calls the item_stack section of code in the controls.xml. You need to duplicate this code in the controls.xml, creating a new control, something like item_stack9 for instance.
Modify this new control item_stack9 so that everything is resized as you like.
Then modify the calling code in the ItemInfoPanel so that it calls item_stack9 instead of item_stack.
The reason other panels get messed up is that multiple windows call the item_stack control, so any changes to that control will affect all the other windows.