const buttonsInstance = (
  <div>
    <ButtonToolbar>
      <DropdownButton bsSize="large" title="Large button" id="dropdown-size-large">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>

    <ButtonToolbar>
      <DropdownButton bsSize="medium" title="Default button" id="dropdown-size-medium">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>

    <ButtonToolbar>
      <DropdownButton bsSize="small" title="Small button" id="dropdown-size-small">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>

    <ButtonToolbar>
      <DropdownButton bsSize="xsmall" title="Extra small button" id="dropdown-size-extra-small">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>
  </div>
);

React.render(buttonsInstance, mountNode);

const buttonsInstance = (
  <div>
    <ButtonToolbar>
      <DropdownButton bsSize="large" title="Large button" id="dropdown-size-large">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>

    <ButtonToolbar>
      <DropdownButton title="Default button" id="dropdown-size-medium">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>

    <ButtonToolbar>
      <DropdownButton bsSize="small" title="Small button" id="dropdown-size-small">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>

    <ButtonToolbar>
      <DropdownButton bsSize="xsmall" title="Extra small button" id="dropdown-size-extra-small">
        <MenuItem eventKey="1">Action</MenuItem>
        <MenuItem eventKey="2">Another action</MenuItem>
        <MenuItem eventKey="3">Something else here</MenuItem>
        <MenuItem divider />
        <MenuItem eventKey="4">Separated link</MenuItem>
      </DropdownButton>
    </ButtonToolbar>
  </div>
);

React.render(buttonsInstance, mountNode);

Update DropdownButtonSizes.js