Provided are an image forming apparatus capable of setting a printing size of a margin by using a preview displayed in an enlarged manner, and a non-transitory computer-readable storage medium. When a pinch-in operation performed with respect to a particular region at a left end in a whole page preview displayed on a display section has been accepted, an MFP displays a left end preview in an enlarged manner to be larger in size than the particular region at the left end in the whole page preview. When a left-to-right swipe operation performed with respect to the displayed left end preview has been accepted, the MFP sets a size specified by the swipe operation as a printing size of a margin of a page as a target of the preview, and displays a whole page preview and a left end preview, both reflecting a margin having the set size.