I finished one of my new Grim Hammers dwarves today (they are going to take longer to paint than I imagined) and am posting this picture of him next to a LotR dwarf. It's hard to tell, because of the heavy armour, but I reckon he is about half a head taller than the older figures.
Now I am very fussy about figures of different sizes but I wouldn't have any problem fielding them in the same force. Things like their hands are exactly the same size, for example. Games Workshop have previewed some more new Dwarves from The Hobbit today so I shall have to get some of them and maybe they will offer a better comparison. The more dwarves the better!
My daughter wants me to get the new Thror figure but my head says that paying £12 for a dwarf is insane but I'm sure I'll get it anyway!