You can do this with CSS, either in the style of a string, or with a class.
With inline style:
<select style="width: 50px;"> <option>item</option> <option>item</option> <option>item is really long</option> <option>item</option> </select>
Class usage:
<select class="narrow"> <option>item</option> <option>item</option> <option>item is really long</option> <option>item</option> </select>
And in your CSS file or nested style sheet:
.narrow { width: 50px; }
Of course, change 50px to whatever width you need.
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