In IIS7 environment, there are several limit for ASP.NET can be override by web.config. One of error could be follow:
The request filtering module is configured to deny a request that exceeds the request content length

This problem occurs because by default in IIS 7 enabled requestFiltering that have MaxAllowedContentLength property. It specifies, in bytes, the maximum length of content in a request. The default is 30000000 (approximately 30 megabytes.)

To override the default setting, you could add following to web.config:

<configuration>
   <system.webServer>
      <security>
         <requestFiltering>
            <requestLimits maxAllowedContentLength="100000000"/> 
         </requestFiltering>
      </security>
   </system.webServer>
</configuration>

refer to:
http://www.element-it.com/RequestFilteringModule-maxAllowedContentLength.aspx

Notice above situation only occur on IIS7 or Windows 2008
———————————–

Regarding asp.net upload size limit, there are other limits like executionTimeout & maxRequestLength. If file upload larger then the limit, "Maximum request length exceeded. " error will be shown.

You can add to the site web.config like follow:
<httpRuntime executionTimeout="seconds"
             maxRequestLength="kbytes"
             minFreeThreads="numberOfThreads"
             minLocalRequestFreeThreads="numberOfThreads"
             appRequestQueueLimit="numberOfRequests"
             useFullyQualifiedRedirectUrl="true|false"  />
ref:
http://www.devx.com/vb2themax/Tip/18803
http://aspnetupload.com/HowTo.aspx

Please note that with higher executionTimeout value your site will be more vulnerable by DOS attack.

Assuming we need to upload 20MB files via script on website, you need to make the following necessary changes.

ASP
settings:
1) write permission to IUSR
2)
AspMaxRequestEntityAllowed = 204800000  – global settings in metabase.xml
Connectiontimeout = 1000 ~ 3000 seconds  – iis entry
AspBufferingLimit – global settings in metabase.xml
in iis
4) Script timeout = 270 seconds – iis entry / server.scripttimeout = 12000 – in code

PHP
settings:
1) write permission to IUSR & Network
2)
upload_max_filesize = 20M
max_input_time = 3000
max_execution_time = 3000
memory_limit = 30Min
file_uploads = On
post_max_size
in php.ini
3) Connectiontimeout = 3000 seconds in iis
4) maxAllowedContentLength = 26214400 for iis7      

ASP.NET
settings:
1) write permission to Network Service
2)
executionTimeout="1200"
maxRequestLength="20480" 
enable="true"
in httpRuntime tag in web.config
http://msdn.microsoft.com/en-us/library/e1f13641(VS.80).aspx

problem when uploading large file
considering memory: http://www.motobit.com/help/scptutl/pa31.htm
pool will be recycled during uploading