The mass is computed based upon the model bounding box itself. So the larger the box the more mass it will have. The Mass Scale scaled it up and down. A value of 2 would be 2x the mass of value 1. So if you set it at like 0.5 it would be half the amount of mass.
Also if you are trying to make a object hover in the air then you are going to need your gravity setting to be much lower then what it defaults to. This will allow you to apply less force upward.