If you are making a poster with web browser screenshots on it, they will look poor quality when eventually printed out at A0 size. This is because unless your screen is absolutely huge, there aren’t enough dots there to look good on a poster. Ideally you should work with vector graphics formats (like EPS, PDF) in a proper drawing program like Adobe Illustrator (since PDF is imported as an ugly low-res bitmap into Powerpoint), but if you can’t do that, then the following may work for you.

1. View your web page in Firefox. Go to File→Page Setup and configure A1 and A2 custom paper sizes (see this Wikipedia page for dimensions). Why they are not there by default (on my machine at least) beats me…
2. Select a large paper size (A1..A3) in Page Setup.
3. File→Print and then hit the Preview button. A PDF preview pops up in the Preview application.
4. If the web page content doesn’t fill the page, kill the preview and go back to Page Setup and increase the Scale percentage.
5. In Preview, use the marquee tool to select the area you need, then Edit→Copy it to the clipboard
6. Still in Preview, do File→New from clipboard, and then in the new window that pops up, do File→Save as… and save as PNG. This PNG will be larger than if you had printed to A4 or letter paper.
7. You should be able to insert the extra large PNGs into Powerpoint, and they should print better than a standard screenshot. Don’t forget to set your Powerpoint document size to the final poster size before you start work!

Note that I haven’t printed on A0 yet, but I see better resolution so far on A4 paper and the screen. It may work best if you don’t resize the images within Powerpoint. You may need lots of memory to deal with the large images.

UPDATE: it looks fantastic in A0. The text in my main screenshot bitmap, which takes up a big chunk of my poster, looks almost vector quality.

Also note that I tried a much easier way (ImageMagick’s ‘convert’ program) to produce large PNGs from the beautifully scalable PDFs produced by Firefox but that failed with an obscure error on the file I had. Much of the hassle would be solved if Preview Save as PNG had some options for output size. But do Apple listen to their users? Not in my experience.