Add the highlighted nodes shown in the image below, location at the top of the screenshots.

Then select the Progress Bar in the BP_InventorySlot widget designer then change the values pointed to in the image in your project to match. This should be the Padding and Alignments.

Next go to the BP_inventory widget and change the Inventory Slot Size variable to X 75 and Y 75 these may already be set like this. You can now use these values to control the size of the inventory slots. Please keep in mind if you set the values to high and the slot may become squished because there is not enough space and you may have to reduce the number of Colums this can also be changed in the BP_Inventory.