by Ken Task.
There is an addon which gives a report of approximate size of backups.
https://moodle.org/plugins/report_coursesize
Get the version of the plugin for your version of Moodle 2.6.
That report is a 'ball park' report for size of the backup.
Also try to get https://moodle.org/plugins/view.php?id=522 - moosh running on your server. It also is command line and a powerful tool.
That has a command for 'course info' you could try to get a more accurate number for backup file size.
Am not a Windows person ... don't think there is a 64 bit version of PHP until 7.0.x and you can't run Moodle 2.6.x with PHP 7 anything ... moodle not compat. Moodle must be at least version 3.0.highest.
Now before you embark on upgrading Moodle, make sure you research that for your platform. Cannot hyperjump from 2.6 to 3.0.x.
If that course did backup successfully, you should then see it by running that query again.
And you should be able to find the file in moodledata/filedir/
You might have to backup that course in two parts ... one backup through one half of the sections. And another that gets the second half of the sections. Then, to restore it, one would have to restore the 2nd backup first, then restore to same course the first backup to get the course completely restored and in the order it was before (I think ... been many moons since I've had to do that to remember!)
'spirit of sharing', Ken