In order to increase the maximum size of files you can upload to your store you will need to change the maxRequestLength property in your Web.Config file. Please follow these instructions:
1. Locate your Web.Config file. You can find it in the root folder of nopCommerce.
2. Edit the file:
Find the httpRuntime tag (e.g. <httpRuntime targetFramework="4.5.1" /> ) and change the maxRequestLength property. If there isn't a maxRequestLength property inside the httpRuntime tag you should add it. In the end it should look something like this:
<httpRuntime targetFramework="4.5.1" maxRequestLength = "4096"/>
Note: If no value is specified for the maxRequestLength property it takes its default value. The default value is 4096kB.
Note: The maxRequestLength can only take up integers from 1 to 2,147,483,647.