To change the height of a SilverStripe HTMLEditorField, all we need to do is use the replaceField method of the Fieldset object. We just replace the field with another instance of itself and we change the $rows parameter. This rows parameter is what is used to calculate the height. The default rows is 30.
This is so easy to do, once you know how, its not funny, but it has taken me months to work out. I always thought there was a bug with the DataObject->toDropdownMap method.