I have taken three Nyala and seen another two taken, so that must make me an expert ....

Seriously though my limited experience is that they are not that big or particularly tough. They look bigger then they are because of all that gorgeous fur. Maybe about the size of a good mule deer.

Three of the five where one shot kills and all three dropped to the shot.
1. 7mm them mag 150 nosler etip (125 yards)
2. 338-06 210gr Barnes X tipped. (225 yards)
3. 270 Win 150 Barnes X tipped (100 yards)

Two required or a least got a second shot.
4. 6.5x284 127gr Barns x LR (75 yards)
5. 338 federal 160gr barns X tipped (175 yards)

I would take the 7mm mostly because it's easier to shoot prone off of a bipod or sitting off of the tripod. Longer shots are possible but in each case we had time to stalk closer.