id say a small. the extra small has issues with having show some boner bulge since the dick is being pressed against by the balls and its smaller.
id suggest a small. its comfy and you wont have to worry about awkward bulges. its relaxed in your pants. and harnesses work great if you wear boxer shorts. if youre a briefs guy it isnt necessary to get a harness because the packer will fit snug. but its all up to you