This is a fully functional corectly propotional 4x4x2
The puzzle is a refined version of the recent v3.1 wich was a bit loose.
Note that the pictures is of the v3.1, but the only diference is on the inside, so they look the same.
The puzzle has a size of 72x72x36mm (18mm cubies) but normal 3x3 stickers will do fine.
To assemble the puzzle so do you need a 24mm 2x2 (Eastsheen). You need to modify that to make it fin inside, see the core picture for more info on how.
More info about this puzzle can be found here