You may notice if you run Get-spfarmconfig you will receive following output
you can change the value of WorkflowBatchSize via set-spfarmconfig
And then if you try to run the equivalent command in stsadm (stsadm -o getproperty -pn workflow-eventdelivery-batchsize) you will notice there is no actual change in WorkflowBatchSize by "set-spfarmconfig"
This is a known issue and one of the properties returned by get-spfarmconfig in powershell is the WorkflowBatchSize property. However,The value of this property is not actually the workflow batch size, it is the work item batch size.Changing this property will change the work item batch size, and have no effect on the workflow batch size.
As workaround to change the workflow batch size,the stsadm command below has to be used:
stsadm.exe -o setproperty -propertyname workflow-eventdelivery-batchsize -propertyvalue <value>