Please see the documentation at http://www.kentico.com/docs/devguide/installation_on_shared_hosting.htm.  

If you're using version before 2.2, you may receive an error message like this:

Exception Details: System.Security.SecurityException: Request for the permission of type 'System.Web.AspNetHostingPermission, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.

You need to upgrade to version 2.2 and adjust the security settings as described in the KB article Kentico CMS in a Medium Trust Environment.

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