The best tool for the job is Paparazzi!, which is a small app that uses the Safari rendering engine. It's very customisable, easy to use, and fast. It feels well-made and well-designed.
Hey, I highly recommend taking a look at Paparazzi! if you're on Mac OS X. Depending on how you've developed your website (materialdesigner has some good tips for you), you can either export a PDF or a full-resolution PNG. I'd export a PDF in this case, especially because it'll contain the vector shapes of any SVGs you use. Exporting to PDF also avoids you having to style the site to actually look good at 7200px wide (the PDF will scale, obviously).
Couple questions (some already asked below):
• Why does it need to be Docx?
• Is this just to save the information? Are you using the information elsewhere, i.e. to plug into a formula or spreadsheet?
• Have you tried attempting to paste it into Word with the formatting intact? Not sure if it works, but they claim it does.
• If you just need a copy (with layout), you can print to pdf. Mac (OSX) has this option built in, and so does Google Chrome. Not sure how it easy it would be to copy info from a PDF it if was required, however. Might be easy enough...
• The above also splits the webpage into letter-size pages. If the webpage is larger than that, you could always opt for something that saves webpages as images. Again, if you need to copy the information later, this might not be the best option, but it would preserve formatting/images, and you might even be able to import into a docx.
• There's just saving as a .htm (webpage). Have you tried saving as a webpage, then opening within Word? Or importing? Then saving as docx?
> Taking screenshots doesn't scale to a large number of pages, even with GUI automation tools, especially when the pages don't fit in your viewport.
I use this free program to make screenshots of web pages when I need them properly trimmed - http://derailer.org/paparazzi/ - and it can easily be scripted. It renders the entire page regardless of size.
Paparazzi will let you take a screenshot of a full web page. If you want to crop it down after than you can just use Preview. (Select the part of the image you want, hit command-k, and save.)