The cyoubx topic says that they have a copyright on the 2x2x2 through 4x4x4 as well as any 2D depictions of them. Their website says they have a copyright on the Rubik's Cube (no mention of other sizes) "in any configuration, [sticker]ed or un[sticker]ed". Their
design trademark specifically only covers a black assembled 3x3x3 fully stickered in the traditional color scheme. It's confusing. I would say that, if they wanted to, they could go after any 2x2x2 through 4x4x4 (or even 5x5x5, I suppose), assembled or no, and your only recourse would be going to court (which you do not want to do). Any cube larger than 5x5x5 is guaranteed to be safe, though.
As has been mentioned, it's still pretty up in the air how much of this would hold up in court. Their patent is expired, their trademark is quite specific, and their copyright makes little sense (how can you copyright a work of art you have not created?).