Sounds like you need to add the following to your Web.config:
<system.web>
<httpRuntime maxRequestLength="2097151" />
</system.web>
This is the setting that restricts the size of files you can upload. The default setting is 4096. Fine if all you ever upload are small files of less than 4MB. I have set it here to 2097151 - the maximum. That is insanely high. Don't leave it at that unless you have a really, really good reason to do so and really, really need to upload 2 gig files.