difference in output between 1x1 tree and 2x2 is roughly 4-5 times per tree. the higher and bigger tree is, the less fertilizer you need. provided enough power, harvesters work FAST, so you might want to consider adding at least 1-2 engines.
you might want to compare heat value of 6 planks to 1 charcoal and go with what have higher value, but i heard that plank has been nerfed recently, so charcoal is probably way to go.
i also highly recommend using induction furnace or two, because of its speed and internal buffer, instead of bunch of powered furnaces.