If you have Views module installed, you can use its views_trim_text() function to programmatically trim field value (or any other text for that matter) to a specified length from your code.
Usage:
views_trim_text($alter, $value)
where $alter array consists of the following elements:
max_length- maximum lenght of the string, the rest gets truncated,word_boundary- trim only on a word boundary,ellipsis- show an ellipsis (...) at the end of the trimmed string,html- take sure that the html is correct.