When system requirements list the disk size requirement, is that the same as download size?

Depends on the game's compression and saved games files. In almost all cases, the download size will be smaller.

Thanks, I was hoping that was the case as a game I want has disk space of 20gb which is a big download that would take a day or two.

will stil be a 12 gig download at the very least

No, system requirements listed are for the game's size on disk after it is installed. Since the installers are compressed, they will expand on-disk as they're installed and thus they are naturally smaller. How much the difference is will vary greatly from game to game depending on what type of assets the game is made up of, and what compression algorithms are used. Another thing worthy to note is that the sizes listed are not measured by automated software that gets it right every time, but rather are sometimes inaccurate due to human error as they're manually updated by humans. A game's disk usage may change over time due to patches, additional content such as DLC additions, etc. It can also vary depending on the block size of the formatted disk, but that's unusual as most disks are formatted with the default block size.

So it's really a ballpark estimate of the disk usage post install.

You can check download size for the vast majority of games on MaGOG. Though new additions may not have it anymore, depends...