Currently the only way to adjust the styles of mdb-select is to overwrite the individual classes in your styles.scss file. Please try to adjust the input to your needs by using following rules:

mdb-select .single {
  height: calc(1.8125rem + 2px);
  font-size: 0.9rem !important;
}

// if you need to change styles of multiselect as well

mdb-select .multiple {
  height: calc(1.8125rem + 2px);
  font-size: 0.9rem !important;
}