How to use checksum to compare file sizes

This is my first time using checksum, and I'm not sure how to proceed this.  Can someone help please?

I've a dtsx package which daily downloads multiple files from an ftp server.  The files have multiple extensions (*.done, *.txt, *.gz, etc.)  I've no problem downloading these files to my server.  However, I need to check the file sizes and checksum.  One of the file that I download is CompleteFile.done; this file contains all other files information and has following data:

CompleteFile generates at 09/16/2008 1:00 PM

FileName          SHA-CheckSum                                                         Size

File1.gz            cf0ae9bf1cb661f080e27852d6f4e1a483360084               637504942

File2.gz            d87f8a91eb15524f0988a1d8f3ad071e3766b2d5              7712174

...etc.

I was trying to save the .done file as .txt and using Flat File Source task to read the data but I was not able to; that's one problem.  Another problem is, how can I use the checksum from this file to validate with the files I already extracted?

Any suggestion is appreciated.

Thanks in advance,

-Ash