The default limit is 100 items in an Exchange 2007 OWA search, if you try to search for something that has more than 100 results it will only display the first 100.
Change the default search limit in OWA 2007
You can change this by modifying the the web.config
The default location is: C:\Program Files\Microsoft\Exchange Server\ClientAccess\Owa\ web.config
Look for the following entry and adjust.
<add key="MaximumIdentityArraySize" value="100" />
And adjust the values to whatever you want, but keep in mind that could have a performance impact on the CAS if there is high use of large queries