The select requires that you set the width of the mdc-selectanchor element as well as setting the width of the mdc-selectmenu element to match. This is best done through the use of another class (e.g. demo-width-class ...).
Unfortunately, unlike the menu, the mdc-select__anchor element is internal to the Angular MDC MdcSelect component.
According to the docs:
Unfortunately, unlike the menu, the mdc-select__anchor element is internal to the Angular MDC MdcSelect component.
Angular MDC version: 5.1.1.