If myriad Hong Kong case manufactures are to be believed, the second generation iPad Mini will be slightly thicker than the first generation iPad Mini. The cases show that the mini tablet will have a thickness of 7.5 mm, which is about 0.3 mm thicker than the first gen model.
If the words of Macotakara are to be believed, width will also increase by 0.2 mm in the new iPad mini. However, since the Mini 2 will be equipped with a retina display, nobody is going to bother or complain about the extra 0.3 mm thickness. This is chiefly due to the fact that more space is need to fit the new display.