An alternate solution to setting domain list widths, is to adjust the width of the field in the form.
For example, this schema has a number of comboBox properties defined as 36 characters:
When the placement metadata is inserted, and Use Form View is turned on, it results in drop-down fields that may be too wide and require scrolling to use.
To resolve this, navigate to Operations > Feature > Dialogs > Form View and adjust the Width of the dialogItems for the Form.
The adjustments will be applied to the dialog in Map:
You may also want to change the width of the formView from its default value of 42 to something more manageable. This change should be made in the Place and Edit dialogs found in the Operations node.
Note: If you prefer to not use a formView, then adjust the width of each dialogItem in the Place and Edit dialogs.
As you can see, it's relatively simple to control the width of your comboBox fields. That said, remember that edits to Operations are overwritten if you insert placement metadata again for the selected feature.