How can I limit the maximum size of the uploaded files?

You can specify max file size by setting max request length parameter in the web.config file. Please open the web.config file, find the <system.web> section and insert the following line into it:

<httpRuntime maxRequestLength="8000"/>

This will increase the max file size to 8MB. You can find more information at: http://msdn2.microsoft.com/en-us/library/e1f13641(vs.71).aspx.
Please see also The IIS6/7 is set by default to allow only 30MB files to be uploaded article.