How much space is needed to reclaim space from a table in INNODB
DISKSPACE FOR EVERYTHING INNODB

SELECT FORMAT(SUM(data_length+index_length)/POWER(1024,3),2) InnoDB_DiskSpace
FROM information_schema.tables
WHERE engine='InnoDB';


DISKSPACE FOR DATABASE mydb BY TABLE

SELECT
    IFNULL(tbl,'Total') table_name,
    FORMAT(SUM(table_bytes)/POWER(1024,3),2) table_size
FROM
(
    SELECT table_name tbl,SUM(data_length+index_length)table_bytes
    FROM information_schema.tables WHERE table_schema='mydb
    GROUP BY table_name WITH ROLLUP
) A ORDER BY ISNULL(tbl) DESC,table_bytes;


DISKSPACE FOR INDIVIDUAL TABLE (mydb.mytable)

SELECT FORMAT(SUM(data_length+index_length)/POWER(1024,3),2) Table_Diskspace
FROM information_schema.tables
WHERE table_schema='mydb;
AND table_name='mytable';


These queries will tell you how much space you need for any table, database, and engine.

SUGGESTION #1

If you would like to simulate OPTIMIZE TABLE just to see diskspace usage, you run the following steps on mydb.mytable

use mydb
CREATE TABLE mytable_new LIKE mytable;
INSERT INTO mytable_new SELECT * FROM mytable;
ANALYZE TABLE mytable_new;
SELECT FORMAT(SUM(data_length+index_length)/POWER(1024,3),2) Table_Diskspace
FROM information_schema.tables
WHERE table_schema='mydb;
AND table_name='mytable_new';
#
# Stop Here. If you are happy with the size, do the last two commands
# If you prefer, drop table mytable_new and do do the online schema change
#
RENAME TABLE mytable TO mytable_old,mytable_newTO mytable;
DROP TABLE mytable_old;


Doing this can give an idea who big the .ibd file will be after running OPTIMIZE TABLE.

SUGGESTION #2

Please do not convert the table to MyISAM. It will be building two files (.MYD and .MYI). Then, you try to converting to InnoDB again. It is essentially the same as doing #SUGGESTION #1. MyISAM is also a little more prone to corruption with no recovery processes in place.

EPILOGUE

By all means, you can proceed with pt-online-schema-change if your data is live. My answer simply gives you queries to estimate sizes of the tables and approximate temp table sizes for OPTIMIZE TABLE.

If you are doing this during off hours, try my post Recover the disk space after deleting rows from table

GIVE IT A TRY !!!