Backup includes/functions.php
Open includes/functions.php
Find:
"cat_description" => format_text($cat_cache[$category_id]['cat_description'], 1),
Add after:
"cat_description_short" => (strlen($cat_cache[$category_id]['cat_description']) > 120) ? substr($cat_cache[$category_id]['cat_description'], 0, 120)."..." : $cat_cache[$category_id]['cat_description'],
Edit templates/YOUR_DESIGN/category_bit.html
Replace: {cat_description} with {cat_description_short}
You can change the 120 to a different number, if the text is too long