By default, your Jenkins workspace would be the one mentioned in JENKINS_HOME. You need to make sure that is owned by the user you specify in JENKINS_USER to have Jenkins access file. So if you wish to change the Jenkins workspace, all you've do is change the path of your JENKINS_HOME.
For slave nodes, specify the default workspace on the slave machine in the slave configuration under Manage Jenkins > Manage Nodes > > Configure > Remote FS root. Again, this should have read/write/execute permissions for the JENKINS_USER user.