The next largest size for Merida frames is an 18inch and when I tried it before I bought this bike, it really felt too 'long' and stretched out. There was no question that I was putting heaps of strain on my arms while riding the 18inch version of my bike. This, plus the handling ability of an 18inch frame could absolutely not compare with the 16inch frame. (which is what I'm riding now)
Incidentally, I also thought about buying a GIANT frame -which are available in abundance here- as they have a 16.5 and 17 inch frame available. However, when I compared the geometry of 16 inch Merida and 17inch GIANT frames, despite the extra frame size, the difference in top-tube length was negligible.
Also, before buying this bike, I was told buy no less than 15 friends who mountain-bike regularly that its better to buy a bike that is slightly small than slightly big.
The logic being that you can always correct your riding position on a smaller bike but an overly large frame is almost impossible to correct for mountain biking.