I have had that problem while moving to IIS7 with different size of files. But solution below worked for me that time. you should add those parts to your webconfig or appconfig file depends on scope you want.
<system.webServer>
<security>
<requestFiltering>
<requestLimits maxAllowedContentLength="524288000"/>
</requestFiltering>
</security>
</system.webServer>
For more information you may look at.
http://www.webtrenches.com/post.cfm/iis7-file-upload-size-limits