Well, it is more elaborate than the FTN95 listing. Also it is free. (2 of your requests)
What it does is the sort of thing I used to do on a listing printout years ago, which is to indicate the 'span' of DO loops (even when they are nested) and block IF statements by lines down the left side of the page. As it is in Fortran, and it has the source code, it could be modified.
If you want more sophisticated, i.e. to draw a flow chart with rectangles and diamonds with linking lines, you may have to either spend a lot of money, or do it yourself by reading the source code you have in front of you.
It isn't clear to me what you mean by huge. 500, 1000, 10,000 lines? How many subroutines? Is it easy-to-read fortran, or the sort of incomprehensible rubbish that used to be written by PhD students with no comments, and 1000 line subroutines? If it is truly huge, with hundreds of subroutines, you might also use a call tree analyser, like this one:
http://drn.digitalriver.com/product/view/id/16268
Whichever way you go, there is no substitute for immersing yourself in understanding the Fortran code itself.
As to whether anything does what you want - you could either specify what exactly you want more fully or give various bits of free software a trial of your own. Alternatively, find your own helpful Fortran expert who is prepared to spend time teaching you how to do it. As your profile does not say where you are, it is difficult for even the most helpful individual to offer ... but if you are in the UK and wanted me to look at your problem I would.
Eddie