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.

What Scott said, and:

• Shift-W will select fit image to Window in no-fullscreen mode.

• Ctrl-H will display image at full size (pixel per pixel) in non-fullscreen mode.

The above are extremely useful when editing - allowing either overall view or detailed close-up.

To maximize large an image to fit your desktop just double click it. (ESC brings it back to where it was.)

Then, like it it's regular window: ...Spacebar will take it to the next image in that directory ...arrow keys or L & R mouse buttons to see the next or previous image in that directory. ... + and - magnifying glass tool enlarges and shrinks image. and much more.

(Here I am, answering a 9 1/3 year old question!)