NOTE – To be able to use this guide you must be using version 1.9.7 of SGK or newer, if you are not then you must follow this DIY Guide fix for this guide to work.
You can go to the BP_InventoryHUD then select the Inventory Widget in the designer view like the image below.
Then in the details the is an option called Inventory Slots Size using this you can change the size of the slots. Keep in mind that if you make the slots to large they may become squished if there is not enough space in the row for the larger slots. If this happens you can reduce the number of Colums this option is also in the details panel when the Inventory Widget is selected. You can also make the Inventory Widget bigger to provide more space for the larger slots.
If you want your slots to be smaller then the default X 75 and Y 75 then you will need to also go to the BP_InventorySlot and then in the class defaults find the Icon Image Size and set the smaller size here as well.
To change the size of External Inventory slots do the same thing as above but select the External Inventory in the BP_InventoryHUD like the image below.