Since I'm the only person that has seen Himumu in real life, I think I have the best judgement on how big his cock is. Judging from his body size etc., a 10 inch long penis with a 4 inch girth would be a baby's arm hanging from his crotch. It would have to have an elbow.
Maybe he's a grower and it's like 5 inches flacid, and balloons up to 10. But I'm putting my money on being disappointed.