How to set size of ui-select?

Could you please tell what is "a good way" for you? If CSS is good enough, then you can style elements with it, would look like this.

.form-input-sm .ui-select-toggle {height: 40px}

That is for simple HTML:

<ui-select theme="bootstrap" ng-model="model">
  <ui-select-match placeholder="Placeholder"></ui-select-match>
  <ui-select-choices refresh-delay="0" repeat="item in roleList">
    <div>{{item.name}}</div>
  </ui-select-choices>
</ui-select>

And I'm not magician, I don't know the selectors just looking at HTML here. If you need to find proper selectors, you can use browser developer tools. The same technique is used to override all default styles in Bootstrap, for example.