ANSWERED ...
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For anyone who still needs this answer (thanks to Rosali for pointing in the right direction), you'll have to make that adjustment in the mail.html template and *NOT* in the mail.css file. You'll find a line like this in the mail.html template:
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Below that, you'll find a portion that reads id="mailboxlist" maxlength="16" /> by default. Those are the amount of physical characters that are going to show from a folders name regardless of how wide you make the pane with all of the folders. So if you want to use longer custom folder names or enable longer folder names for your users, then just change the 16 in maxlength to 25 or whatever. Don't go overboard though because then the block with the folders could get kind'a ugly.
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Take a look here (http://www.roundcubeforum.net/deutsch/4767-verruckte-grafiken-ist-hier-los.html). It is always the same approach. RoundCube strictly separates: core, html, (core)js and css.
Q: where? A: css!