So, lets say you want to print web pages as they appear on the screen when displayed at 1024 pixel width. To set this up the first time you do it, you need to create a custom paper size with width 1024/27 = 38cm, say 40cm allowing for 1cm margins. You want to keep the same shape as your actual paper, so for A4 paper you want to use a length of 40*1.4 = 56cm. From within Safari, go into Page Setup, and select Custom Paper Size from the Settings drop down menu. Hit New to create a new paper, call it Safari1024, and specify height 56cm, width 40cm, and all margins 1cm. Click Save to store this, and then OK to exit page setup. Go back into Page Setup again, and set the Paper Size as Safari1024 (which should now be at the bottom of the Paper Size drop down list). This setting should remain in place for Safari until you change it.
Whenever you want to print a page, go into Print in the normal way, but click Preview rather than printing directly. Preview should show the page formatted as if displayed in a 1024 pixel wide window. Now from within Preview, check that the Page Setup is set to the actual paper size in your printer (A4 in this example), and then still within preview select Print and this time click the Print button to do the actual printing. Preview will automatically shrink each page down to the size of the paper selected.
Even better, if you use Acrobat Reader as your Preview application (set Acrobat Reader as the default application for .pdf files), then while you're previewing a page, you can use the Graphic Select feature to choose any rectangular area within the page to print instead of the entire thing. So you can miss out all those annoying adverts and side panels on the printout.
You can, of course, set up alternative paper sizes for printing pages at pixel widths other than 1024 -- you just need to remember to select the paper you want in the Safari Page Setup before you print. Note that the larger the pixel width you squeeze onto the paper, the smaller the text size on the printout.
[robg adds: I believe this is fixed in the newer Safari in 10.3; at least, I haven't had any issues with output size when printing to a Brother Laser from Safari.]

