Listbox in the backend

I am developing a plugin. In the plugin I have a form. In the form I am trying to make a list box with the following code:

<select style="width:150px" id="input_10" name="items" size="4" multiple="multiple">
                        <option>  </option>
                        <option value="Option 1"> another option </option>
                        <option value="Option 2"> Option 2 </option>
                        <option value="Option 3"> Option 3 </option>
                    </select>

but when i go to the form it is sized as if it was 1. and there is no scroll bar.