I took a look at the Breadcrumbs snippet and found this on line 414:
$pretemplateCrumb .= '<a class="'.$crumbClass.'" href="'.$modx->makeUrl($c['id']).'" title="'.$title.'">'.$text.'</a>';
So, it looks like the $text variable contains the data you want to truncate. You could add the following right before line 414 to truncate it. I
haven’t tested this but should get you pretty much there:
if (strlen($text) > x) {
$text = substr($text, 0, x);
$text .= "…";
}
Just replace x with the number of characters you want to display in the link.
EDIT: On a closer look you might want to add the code before line 390 - that way $text will be updated if you’re using a link or a span.