I re-decked using the same size (2x6) so I could use the drilled out holes already in the trailer frame. Many people will just use self-tapping screws to attach the deck without re-using the same holes though.
I also borrowed DiamondPilot's 'fat buddy' trick to place down the decking and avoid cutting into the frame. I did it solo so I had to modify the procedure but it worked out well. My trailer is 6.5 x 10.