Automatic Resizing in Irfanview

Go to View > Display Options (window mode) and select Fit images to Window or Fit Only big images to window.

If you maximize the IrfanView window, the first option (Fit images to Window) should suffice. This will scale up smaller images, and scale down the larger ones.

If you keep IrfanView windowed, then the second option will only rescale images that are larger than the IrfanView window. Images that are smaller will display at their native resolution.