That three inch toy holocron looks larger in your hand than the one in the show does in Maul's, even presuming that your hand and Maul's are the same size. The 2"per side cube fits the palm better. It is actually palm sized , unlike the 3" per side cube. If you look more closely at the screen capture of Maul holding Kanaan's holocron, it doesn't sit within his palm the way he's holding it. As with your 2" box, it fits within the crease of his fingers. so he is holding it only by the fingers . It is not resting in his hand. And, when you do let both cubes rest in your palm, it is the 2"x2"x2" cube which fits perfectly within the palm, not the 3"x3"x3" cube. Thus, the 2"per side cube is more accurate. But, as I said, some holocrons are larger, such as the Great Holocron, but the basic standard size is about 2"--small enough to fit perfectly within the palm of the hand .
The Replica Prop Forum is the best place in the world that I know of where these kinds of things are discussed and shown off. The guys who make the actual props for the actual movies are frequently found in these forums — Like Adam Savage, the guy who made the original Thermal Detonator.
The best example I’ve been able to find for this particular holocron can be found in the thread at http://www.therpf.com/showthread.php?t=245142&p=3734733&viewfull=1#post3734733
Note that his design is also a 3” cube. Most other cubic holocrons on the forum are larger, some are considerably larger.
There’s another take on the same design in the thread at http://www.therpf.com/showthread.php?t=263551&p=4023170&viewfull=1#post4023170
You’re welcome to have whatever standard size holocrons you want in your game, but I’m confident that there is enough information here to prove relatively conclusively what size they typically are, because that’s the size they’d have to be in order to get the shapes and designs and electronics into them in order to work that way.