I am not sure this will help you because I have never used IrfanView, but I have used Photoshop which has an image resizer.
Try finding somewhere the option "Scale Styles" or "Constrain Proportions" or something of similar meaning and untick it. Those options mean that when you type in the width, the height changes so that the picture doesnt become deformed and when you type in the height the width changes again, for the same reason.