Probably a t25 torx.
Now, when removing the wheel, you really need to be careful, as I ended up with a broken nose when I first did one, 5 or so years ago.
Once the airbag is completely out of the way, and all you have left is the large nut in the centre, undo it all the way off, and then at that point screw it back on one full turn.
The reason behind this, is that steering wheels come off so rarely, that they are almost always stuck to the steering colomb.
If you remove the nut completely, and then pull and tug at the steering wheel, you'll need that much force to pull it off, that when it does release, it'll end up doing to you, what it did it me. Breaking your nose!
If you leave the nut on a turn, it wont fly off and sock you one in the face.