Given the fact that plants cannot do photosynthesis at night but need respiration for their energy needs, they use up oxygen and generate carbon dioxide. But how much is this? If I fill a room with plants thoroughly, what composition will the air in this room be in the morning? Are different plant species vastly different in this regard?
Roughly, half of the CO2 assimilated annually through photosynthesis is released back to the atmosphere by plant respiration (Gifford, 1994; Amthor, 1995).
Furthermore, it appears like plant respiration rates do not change much with changing temperature, and that larger plants have higher respiration rates.
Your information is wrong. Plants do not use up oxygen and release carbon dioxide it is the other way around. When people put plants in there households the plant use the carbon dioxide we release and transfer it into oxygen. This is one reason people don't want the rain forests cut down because plants cleanse the air. So the composition of the air may be humid (because off the water from the plants) or it may be a lot easier to breath. And to answer your last question yes they are. If you plant a small tree roughly 8 feet. The photosynthesis of that plant would be a lot stronger than if you where to plant a rose.