Is there a sketch for this, or any kind of plan for the final composition? I've shot myself in the foot a few times jumping into projects with no real direction a few times too many. I say get your terrain details to be correctly proportioned to each other, the guy is the least of your worries, if the grass is too big for the trees or the pebble texture at the river bank then it's not going to matter how your guy is scaled, it just won't look quite right.