- Set up a really freaking big custom paper size:
- Hit Cmd-Shift-P to bring up the Page Setup panel.
- Select Custom Paper Size from the Settings drop-down menu.
- Create a new paper size with obscenely large dimensions (something on the scale of 99"x99").
- Select Page Attributes from the Settings drop-down menu.
- Select your new paper size from the Paper Size drop-down menu.
- Save your web page as a PDF:
- Hit Cmd-P to bring up the Print panel.
- Click the Save as PDF... button.
- Specify a n/s/l for the file and click Save.
- Crop the image:
- Open the resulting PDF in Preview.
- Click the Select Tool icon in the toolbar (or the Crop Tool icon if it is there and save yourself some keystrokes).
- Select only the part of the PDF that you want to save.
- Hit Cmd-K to Crop the image.
- Hit Cmd-S to Save the image.
[robg adds: A couple other things to consider. One, if you do a lot of this stuff, get SnapWeb. It's $15.00 (or $30.00 for commercial use), but it uses Safari's rendering engine and makes it simple to save a web snapshot without any page breaks. Second, when I was testing this, I had troubles if I increased the page width -- sites that have infinite-width tables would render on the printed output as super wide. Instead, I just changed the page length and left the width alone.]

