I have always used 1x4's at various lengths for leveling the base gravel of patios since the 3.5" board is just about the height of the sand and pavers and makes a good guide for going around edges.
I have tried 2x4's but seemed to be too heavy and wouldn't screed the gravel as well.
When I level it with the board I hold a 4' level to the board to either make it level or to make sure that it's sloping. I usually have the bubble part way over the line to have it slope if there isn't a specific slope.
It does take awhile but that's the way I have always done it and had to teach myself. Maybe someone has a faster, better way.