It would be big, if we base it on the AK-50 design made by Brandon Herrera, aka "The AK Guy" on YouTube. The AK-50 is a real steel firearm that uses the Kalashnikov design but made to fire a .50 BMG round. That's one massive round that needs a big gun just for it to be fired. Airsoft Al talks about what if an airsoft version is made in this video.

Below is a video the AK-50 project by Brandon Herrera: