I've decided to draw the oliphaunts not too much larger than some real-life extinct relatives and ancestors of elephants, but still big enough to be considered absolutely massive. I've mixed traits from mammoths and modern elephants, but I also wanted to give them an extra set of tusks to make them look more fierce and ancient.
Did you see my rendition of the fell-beasts?
I just now had a look at them. Not how I would have drawn them, but I like how the fell-beasts look like they could be real animals.
Well done! Aside from the extra pair of tusks, it looks very much like a mammoth-elephant crossbreed!