What blue would be good for a medium-sized room - light or dark?

There are hundreds of Resene blues available and they all need to be judged according to the mood or ambiance they create and how they react with pre-existing colours in the room. Also, most importantly, what the quality of natural light is.

You might like to check out the following:
For light blues, try Resene Half Escape or Resene Breeze;
 and for mid tone try Resene Meltwater, Resene Hemisphere or Resene Zephyr.