How to use check box in select list for multiple selection in content type?
You can simply achieve that by following way.

Choose Field Widget Type 'Checkbox' instead of Select list.

If you want to make appear checkboxes field as multiple select list,Add the following line in your CSS file

   #field-id-name{ height: 100px; overflow: auto; width: 100px;  }


Note: replace field-id-name with id name of the field; change the width and height according to your requirement.

After applying CSS, the checkbox list field in content type appears as follows.



I hope this simple solution helps