Unlike some other remote desktop sotware, DWService does not rely on the RDP protocol. The RDP protocle creates a virtual graphic session which offers, for the remote desktop, the same resolution of local computer.
When accessing a remote desktop through DWService, you will be presented the same resolution as the remote desktop. Therefore to setup a different screen resolution through DWService, you will need to adjust it through the remote desktop settings.
Note that depending on your configuration, if you use a server without a screen you may not be able to adjust it.