re:
!> Is that possible via the Web.Config?

In web.config...

<httpRuntime maxRequestLength="8192"/>

...will limit uploads to 8MB in size.

Juan T. Llibre, asp.net MVP
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In my case I changed it in my Machine.config
But I think if you put it in web.config then it overrides the one in
machine.

Stolen instruction from   http://forums.asp.net/p/1048294/1948278.aspx

One of the great things about .NET, however, is that it usually provides a
way around limitations. You can usually change the default settings that are
in place. To change this size limit, you make some changes in either the
web.config.comments (found in the ASP.NET 2.0 configuration folder at
C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\CONFIG) or your application's
web.config file.

In the web.config.comments file, find a node called <httpRuntime> that looks
like the following:

<httpRuntime
executionTimeout="110"
maxRequestLength="4096"
requestLengthDiskThreshold="80"
useFullyQualifiedRedirectUrl="false"
minFreeThreads="8"
minLocalRequestFreeThreads="4"
appRequestQueueLimit="5000"
enableKernelOutputCache="true"
enableVersionHeader="true"
requireRootedSaveAsPath="true"
enable="true"
shutdownTimeout="90"
delayNotificationTimeout="5"
waitChangeNotification="0"
maxWaitChangeNotification="0"
enableHeaderChecking="true"
sendCacheControlHeader="true"
apartmentThreading="false" />
A lot is going on in this single node, but the setting that takes care of
the size of the files to be uploaded is the maxRequestLength attribute. By
default, this is set to 4096 kilobytes (KB). Simply change this value to
increase the size of the files that you can upload to the server. If you
want to allow 10 megabyte (MB) files to be uploaded to the server, set the
maxRequestLength value to 11264, meaning that the application allows files
that are up to 11000 KB to be uploaded to the server.

Making this change in the web.config.comments file applies this setting to
all the applications that are on the server. If you want to apply this to
only the application you are working with, apply this node to the web.config
file of your application, overriding any setting that is in the
web.config.comments file. Make sure this node resides between the
<system.web> nodes in the configuration file.

Another setting involved in the size limitation of files to be uploaded is
the value given to the executionTimeout attribute in the <httpRuntime> node.

The value given the executionTimeout attribute is the number of seconds the
upload is allowed to occur before being shut down by ASP.NET. If you are
going to allow large files to be uploaded to the server, you are also going
to want to increase this value along with the maxRequestLength value.

One negative with increasing the size of a file that can be uploaded is that
there are hackers out there who attack servers by throwing a large number of
requests at them. To guard against this, you can actually decrease the size
of the files that are allowed to be uploaded; otherwise, you may find
hundreds or even thousands of 10 MB requests hitting your server.