To set the value of a Action Form dropdown using JQuery
If you place your code in the On Change/Click section of a form field, the following will work:
form.fields.FieldName.value = "AValue";
*Note the value you are setting must match one of the choices in the dropdown
If you are placing your code in a static text field or the initilization section,
you might need to get in-scope to set it. Try the following example
//gets the scope and uses it to set the value
angular.element($("#dnn931Size")).scope().form.fields.Size.value = '12px';
//Update the UI with the change
angular.element($("#dnn931Size")).scope().$apply();