Man, was this a pain to find. I’m assuming some proficiency with XML and Quicksilver here; leave a comment if you need more specific directions.

Install the Web Search module from the Quicksilver plugins pane if you haven’t already. Quit Quicksilver. Then edit this file:

~/Library/Caches/Quicksilver/Indexes/QSPresetDocWebSearches.qsindex

Search for:

<string>qss-http://www.google.com/search?hl=en&q=***</string>

And change it to:

<string>qss-http://www.google.com/search?hl=en&q=***&num=100</string>

Be sure you’ve got the “Google Search” entry; there are many similar ones. I originally edited the file while Quicksilver was running and had to quit and reload the catalog several times before I got it to stick. Your mileage may vary.

To test, bring up QS, type “Google Search”, tab twice, type your query, and press Return. Your default browser should load a Google Search with 100 results per page. You can of course make any other tweaks to this or any other search URL that your heart desires.