mysql: find column data size on disk
Here is how you find the sum of the lengths the LONGTEXT column of all rows

SELECT SUM(field_length) TotalFieldLength FROM
(SELECT LENGTH(LongTextColumnName) field_length FROM mydb.mytable) A;


Once you delete the rows you want, just compress the file

You can do it in a single command

OPTIMIZE TABLE mydb.mytable;


or in stages

CREATE TABLE mydb.mytablenew LIKE mydb.mytable;
ALTER TABLE mydb.mytablenew DISABLE KEYS;
INSERT INTO mydb.mytablenew SELECT * FROM mydb.mytable;
ALTER TABLE mydb.mytablenew ENABLE KEYS;
ALTER TABLE mydb.mytable RENAME mydb.mytablezap;
ALTER TABLE mydb.mytablenew RENAME mydb.mytable;
DROP TABLE mydb.mytablezap;
ANALYZE TABLE mydb.mytable;


Give it a Try !!!