When publishing a Forms Workflow, there is nowhere to set the size of the Window that opens unless "Is Web Part" is selected in the Service Catalog Settings.
This is currently a product limitation. It has been reported to Development and this functionality may exist in a future version of Workflow.
There are two possible solutions to this. If you are willing to make the Service Catalog Item accessible to the Web Part Service Catalog (recommended), use option 1. Otherwise option 2 will work.
After making changes, restart the Symantec Workflow Server Service as well as IIS to enforce the changes become active.