I have a need for my site's visitors to print line art to a certain size. I experimented with creating a PDF with said line art, published it on my site, and printed it out using two different printers and they printed to the desired size. But I realize that doesn't mean it will work for everyone.
Can I rely on the user being able to always print out the image to the correct size? If not, what are the variables? Any way to do this reliably?
A pdf will always print to the right size - that is the beauty of the pdf format - things stay the way you published them. In fact, that is probably the only reason this annoying format has any staying power.
Just be sure your line art is butting against any of the page margins where a printer might not physically be able to print (ie. keep margins at 1 inch all the way around) and you will be fine.