Are you sure that a drop down menu is the best option for what you're doing? Maybe another Object type would suit your purposes better--like radio buttons.
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Anyway, You can edit the XML source and add a CDATA tag around the items in your list. So, if you have a drop down list with {Something really long goes here, Something else really long here, And another really long thing} as your options, click on the XML Source tab.
Add
<![CDATA[Something really long
goes here]]>
Now, this will break the line as you're wanting to do, but it's going to cause a different problem--The list of items was originally selected by the first, second, and third lines in the dropdown list. They still are. So, you have 6 different lines, and only the first three can select something. If you're just wanting to have a dropdown that the user selects to read a list and not to select something, then this is probably ok. If you want the user to select an item, then this is a terrible idea.
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If your lines are just a little bit too long to fit, you can set the value's font to "0" Then it will shrink and expand the font size to fit the largest selection. Two notes, the dropdown arrow will cover the last bit of the line, and there is a limit to how small it can get.