I have a Concord Ceilng fan model HC-1131. It has 16 inch blades but I want to move it to a larger room. Can I install 21 inch blades on it?

Have you tried checking your manual that came with the fan?

Absolutely not. These fans' motors are sized for the length blades that they have. You will need a motor size to match the blade length.

Even if the blades are lighter??

It has nothing to do with the weight of the blades, it's the power needed to drive those blades. Blades are designed with a certain pitch and length to move so much air and you cannot make a set of blades designed for a 1/3 HP motor work with a 1/5 HP motor. In order to make a set of blades move twice the amount of air you have to cube the horsepower and square the speed (on a squirrel-cage fan, anyway) which would cause everything to fly apart. Fans are very specific about how fast they are designed to go and the HP they need to drive them. If the motor's HP is the same for the longer blades then you can switch blades, but check it out, first.