if you have some sql and are selecting a bunch of data, you can take a substring of the whole string using php's [man]substr[/man] function:
$sql = "select * from myTable";
$res = mysql_query($sql)
or die('query failed');
while ($row = mysql_fetch_assoc($res)) {
echo substr($row['longTextField'], 0, 25); // echoes first 25 chars of the string
}
If you want to do it in sql (which might save memory or something i don't know) then i think this might work:
select substring(longTextField, 1, 25) from myTable