When using an attended robot [Studio or Attended] jobs are not triggered from Orchestrator, they can only be initiated by the User on the local desktop where the Attended Robot is installed.
Orchestrator is used to provide the Nuget packages to the machine hosting the attended robot assuming the user logged in has permissions to the folder and process that was published. In addition to licensing the attended robots and handling the interaction for any Logs, Queue Items, Assets, etc.
Please see the following documents for recommend/minimum sizing from UiPath
If you are going to be installing Robot on a Server for a VDI setup, etc. The sizing will largely depend on what is being automated and how many concurrent users will be using the system at any given time, the footprint of the Robot service is pretty minimal.