Size and Weight:
About the Irish Elk (Megaloceros):
First things first: although Megaloceros is commonly known as the Irish Elk, it's important to realize that this genus comprised nine separate species, only one of which (Megaloceros giganteus) reached dinosaur-like proportions. Also, the name Irish Elk is something of a misnomer, as Megaloceros had more in common with modern deer than American or European elks.
Now that that's out of the way, the Irish Elk (at least Megaloceros giganteus) was the biggest deer that ever lived, about eight feet long from head to tail and weighing in the neighborhood of 500 pounds to half a ton. What really set this megafauna mammal apart, though, were its enormous, spreading, ornate antlers, which spanned almost 12 feet from tip to tip and weighed just short of 100 pounds. Why didn't these top-heavy antlers cause Irish Elk males to tip over? Presumably, they had exceptionally strong necks (not to mention a finely honed sense of balance).
Sadly, the majestic, giant-horned Irish Elk vanished off the face of the earth about 10,000 years ago, probably hunted to extinction by early humans.