I think we used 4 inch long to connect the 2x4 to the 4x4. Not sure what exact size though, my dad went to the hardware store without me for them, I'll look and see if I have a box or anything that says when I get home in the mornin.
Tips... it really all went together pretty well, we used those plans but ended up tweaking little things here and there as needed to it work. It deffinately helps to have at least one other person there though,
If I think of anything I would have done differently or any tips that might help I'll add em here.