input[id^='id_']{width:200px;} should work. It certainly does in this fiddle:
http://jsfiddle.net/jYZnX/
EDIT: Also, to show that it doesn’t pick an input without an id beginning 'id_':
http://jsfiddle.net/jYZnX/1/
EDIT 2: As your Document Mode seems to be set to Quirks this will cause issues with the css selector. Set your doc type correctly, eg using <!DOCTYPE HTML>. This will need access to the original code for the web pages though, so without that you will be in a spot of bother.