I have a customer that would like to add storage space in his garage attic. The attic has 2X4 trusses. The garage is 24 X 30. I have a nice space from one end of the attic to the other to add a floor. My question is: do I need to beef up the ceiling joists to gain some strength for the added weight? Would 2X6's be enough? What would be a good flooring material for this project (OSB, plywood)?

Thanks for any advice in advance!