I am finishing my attic in a 100 year old house. They used 2 X 6's for the attic floor joices running 16 feet (the opposite way of the roof "joices"). Of course this is not strong enough for a floor. There is a lot of bounciness to it causing cracking ceiling plaster below. My question is can I use 2 X 8's for sister joices (I would also attach 2 X 2 strips along the tops of the existing 2 X6s to make them flush) or do I really need 2X10's for a span of 16 feet? I would like to raise the floor as little as possible due to headroom in certain areas of the attic. Thanks.

If you are making a living space in the attic you should be installing 2x12. Standards have changed and those attic joists are not floor joists, they are ceiling rafters that you will make into a joist when you build your new room.

check with your local suppliers, they will have a rating chart to help you. I don't install many 2x8's , 2x12's , etc... any more. You can get much more support and further spans out of a TGI and it usually is cheaper or comparable.
Mike