think if you check the size of everything except the FreeSpace2\Installer directory, that should work... I think.
That whole folder is text files anyways. It won't significantly affect the total size.
You DID say "Install All." That's all of FreeSpace 2, the very large content upgrades known as mediavps, all of FreeSpace 1 and Silent Threat, Awakenings, Derelict, JAD 1&2, FreeSpace Demo Campaign, the cutscenes for both games in two different formats, All the multi missions and voice files, etc.
What I'd recomend is making sure you delete any .mve files you have in the directory or its subdirectories, as well as the files at these locations:
freespace2\FS2MVEcutscenepack.vp (NOT freespace2\FS2OGGcutscenepack.vp!)
freespace2\fsport\FS1MVEcutscenepack.vp (NOT freespace2\fsport\FS1OGGcutscenepack.vp!)
This will free up quite a bit of space, and you won't be losing any needed files.
Also, any zip files left behind by the Installer can be safely deleted.