How do we control Approval manager list drop down while Sending record for approval?

We can do this by going to setting page of particular process ‘Workflow Management’ ->States,. In this page, you need to click under ‘Customize fields’ column for a specific state name for which you want to control Approval list. You can control the drop-down list on selecting the roles in ‘Roles to display in filter’. In case if the state is inherited from approval state you will get only Approval manager role listed in ‘Roles to display in filter’.

Please refer our online help link for more details - States