VB6: Determine filesize of MS Office document by reading its contents

Project Description:
To see why I need this project, imagine a 10mb file full of random junk bytes ... but there's a small 2mb Office .DOC file somewhere near the middle, although it has junk data both before and after it.

LOCATING the Office document is easy ... simply search for the "ID/Magic" header that Office files use at their very start: 0xD0CF11E0

But now that I've located that I still need to "walk through it" to add up the size of each subsection of the .DOC file to calculate the overall FILESIZE, which is what I'm ultimately after.
And this is what I need you to do :-)

Please note the code must be in VB6, and not use any OLE or Office-related API calls or 3rd-party controls ... in other words, it just needs to use regular file i/o and string-related functions to do it's "walking" through the file.

Ive found these pages useful, but Office internals is all new to me so I hand it over to you!
http://blogs.msdn.com/b/openspecification/archive/2013/01/16/determining-office-binary-file-format-types.aspx
http://www.openoffice.org/sc/compdocfileformat.pdf