To change the image size of the inventory slot go to the BP_PlayerInventory then in the class defaults search for InventorySlotSize.
Using the InventorySlotSize you can set the size (In pixels) of the inventory slot image, the X and Y MUST BE kept the same value so the slots stay square.
If you are trying to change the size of slots in the custom grid widgets you will have to go to the BP_InventoryCell and change the Cell Size in its Class Default and go to the BP_InventoryItemIcon and in its details change the Icon Size as well. Make sure you set these to the same sizes in both widgets.
Note – If you cannot find the InventorySlotSize in the class defaults then search the My Blueprint panel in the BP_PlayerInventory and select the variable there and change its default value instead.