probably a silly question, but have you remembered to save the image when you've got it the size you want? In theory, you can adjust the image size to whatever you want on Photoshop (I don't know Irfanview) and save it, and that's what you get when you print it out. I've nto had trouble doing so.
Also use the 'print preview' function if you've got one before printing, to see how it will look.



