To answer your question, and get the Prefix
too, for MySQL you can do:
select Prefix, CR, length(CR) from table1 order by length(CR) DESC limit 1;
and it will return
+-------+----------------------------+--------------------+
| Prefix| CR | length(CR) |
+-------+----------------------------+--------------------+
| g | ;#WR_1;#WR_2;#WR_3;#WR_4;# | 26 |
+-------+----------------------------+--------------------+
1 row in set (0.01 sec)