My Asus Chromebox only does 2242, which I haven't seen in large capacity from reputable vendors. Maybe they increased the length for future Chromeboxes though.
You probably have the previous generation, the one I got is a chromebox 3, link to picture of MB provided above, you can check the SSD spot, the stand-off for the mount can be moved to an alternate position for a longer SSD on that one.
Be very careful when moving that stand off so you don't damage the circuitry.
Indeed, and not that much money either. But even if the longer screw position isn't there it usually is there electrically and you might be able to run a bigger unit anyway by removing the stand-off and using some double sided tape+a tie wrap. Not the most elegant solution but hey, this is hacking :)
You can easily get those in 2TB if you want.