Controlling the size of printing.

I need to scan a line art template and prepare it in such a way that when someone on the web right clicks and chooses print, the image will print at the original size.

An example:

This image is 800 px by 800 px and I want it to print 4.5" by 4.5". If the image is printed at this size exactly the pattern will be at 100% scale.

Should I set my canvass size at 8.5" x 11" or something else, like 4.5" x 4.5".

Printing from windows at 4.5" x 4.5", the document is forced to fill the whole 8.5" x 11" page and is larger than the original scale. If I place this image on an 8.5" x 11" sized document the image is scaled to fit and is to small. I can get the correct size by printing from Photoshop or Lightroom but most people will not have those programs available. I need something that will print exactly to the scale needed automatically for most printers when someone only sets the page size at 8.5" x 11" and clicks print.

What should I do?

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Newbie, having fun.