Dumb question, but somehow interesting. Is it just thr $_GET-limit or do they have some other techniques ?
2048 is the right answer
<input autocomplete="off" class="lst" type="text" name="q" maxlength="2048" value="" title="Search" spellcheck="false">
hope so it will help you.
For the Google Search API:
There is a limit of 320 characters returned for each meta tag when using getfields.
Usage
GET /search?q=[search term]&output=xml&client=test&site=test&getfields=[meta tag name]
http://code.google.com/apis/searchappliance/documentation/50/xml_reference.html
LOL. check the HTML input maxlength on google homepage.
<input autocomplete="off" class="lst" type="text" name="q" maxlength="2048" value="" title="Search" spellcheck="false">