It should be controlled by application server - in this case IIS. Files are uploaded to "/{secureUI}/cms/admin/" location in CMS.

So to change allowed upload file sizes you would need to change web.config section for that location - either "/{secureUI}/cms/admin" or just a "admin".

<location path="Admin">
    <system.web>
      <httpRuntime maxRequestLength="...." />
    </system.web>
    <system.webServer>
      <security>
        <requestFiltering>
          <requestLimits maxAllowedContentLength="..." />
        </requestFiltering>
      </security>
    </system.webServer>
  </location>

By default it should be around 28MB.

http://www.iis.net/configreference/system.webserver/security/requestfiltering/requestlimits

Tried adding these values in web.config to both the admin path and global files path without any luck....

With these settings I could managed to upload 600 MB file to Global files folder running under IIS 7.

<location path="cms/CMS/admin">
  <system.web>
    <httpRuntime maxRequestLength="2147483647" executionTimeout="3600" requestValidationMode="2.0" />
    <authorization>
      <allow roles="WebAdmins, Administrators" />
      <deny users="*" />
    </authorization>
  </system.web>
  <system.webServer>
    <security>
      <requestFiltering>
        <requestLimits maxAllowedContentLength="1073741824" />
      </requestFiltering>
    </security>
  </system.webServer>
</location>

What is the error you see?

Just tried these above settings and uploading a 300mb file - getting "timed out, or max file size exceeded". Nothing recorded in the error logs. Had a Google around and tried a few different solutions..