What length of screws to use over 3/4" plywood
If it were me and i was going to put additional underlayment down on top of the new 3/4 ply, i would use at least 2 1/2" screws and do my best to find the floor joists below. if you are using the new ply directly as underlayment i would use 1 1/2 screws to attach to decking.