The html code for a drop down box for size is:
<select name="txSize_999999"> <option>Small</option> <option>Medium</option> <option>Large</option> <option>X Large</option> <option>X X Large</option> </select>
For colors you might use
<select name="txColor_999999"> <option>Black</option> <option>Red</option> <option>Green</option> <option>White</option> <option>Blue</option> </select>
Note: It is very important to use the precise 'name' attributes you see above. If you add this to your products and do not see the drop down box on your item detail page (item.wws) please contact support. Most likely you are using a custom version of the page and your custom version does not have the tag <%=oitem.sizes%> or <%=oitem.colors%> in it.