We recommend that your dock should be built with 4x6 lumber on the perimeter of your dock. Your stringers and crossmembers generally can be 2x6 lumber. However, if you are fastening 2 floats onto one stringer, you may want a 4x6 at that attachment point, as a 2x6 doesn't give much extra room.
Docks designed with ThruFlow decking have stringers spaced 15"-18" depending on the size. Trex decking must have stringers spaced no more than 12". 2x6 decking lumber may have stringers at 24" apart.