The sprite in question isn't exactly large by todays standards, relatively speaking. The size of your viewport doesn't really matter as far as the device is concerned. However it might cause memory issues if you have a lot of animation frames (it also might not, it really depends on how many frames it contains). Your idea of splitting up the boss into multiple pieces is a good idea and a fairly common technique to optimize memory usage in other games. If there are significant areas that do not need animation, it can make a huge difference in memory use, especially as your frame count increases.
Running out of vram can cause your game to either fail to load, crash, or otherwise run at extremely low fps.