Citrix Metaframe&High Res Client Display

I am running a Citrix Metaframe Server 1.8 on NT4 Terminal Server, but when trying to connect a client running Citrix Client I get a licence error when configuring the client to run at 1024×768 with pref 16 bit or 24 bit colour to enable use of graphic design software across an existing VPN. Is this a configuration issue or do I need different licence options for this higher resolution?