Installing OS X on new SSD with 10.8 found online Since I had lost the CDs that came with my MacBook Pro (late 2010/11 model – 10.6 OS X CDs) I couldn't continue with my OS X installation. I asked Apple about purchasing the CDs but it would take over 2 months for them to arrive :/ (I can't wait that long as I use it for work.)

I have found a version of "Mac OS X 10.7 Lion (Final Retail)" online but it's only 3.5 GB. Is it a logical size of the OS files to be that "little"? I remember OS 10.6 was like 6 GB+.

If I went on and burned that dmg file to a disk would I be able to install Mac OS X or is "Mac OS X 10.7 Lion (Final Retail) - 3.5 GB" a patch file and I would need the operating system to be already installed?

What I am worried about is the reliability of the uploader of this file and if he has tampered with it.
Use the checksum to validate the integrity of the file.

Sometimes when you download large files, the host will provide an MD5 checksum, which is a hash of the raw file data that you can use to compare and see if any of the data was corrupted, changed, or lost during the download process.

To verify the MD5SUM in OS X, simply open the Terminal and type “md5″ followed by one space, and then drag and drop the downloaded file from the finder into the terminal window. 

Hit return on your keyboard and wait a few moments (The larger the file, the longer it will take). 

The terminal will spit out the MD5 hash for the file, which you can then compare to the once provided by the download originator!