Here is an example on how to get size of a blob entry in a mysql table:
mysql> describe mytable;
+-----------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| mynumber | int(11) | NO | PRI | NULL | |
| mystring | varchar(255) | YES | | NULL | |
| myboolean | tinyint(1) | YES | | NULL | |
| myblob | blob | YES | | NULL | |
+-----------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
mysql> select mynumber,sum(length(myblob)) from mytable;
+----------+---------------------+
| mynumber | sum(length(myblob)) |
+----------+---------------------+
| 101 | 5888 |
+----------+---------------------+
1 row in set (0.00 sec)
