Maybe not a harder question, but I'll leave it for you to do the math. I got out my Ames M1850 Foot Officer's, and from the pommel end (butt of the hilt) to the bottom of the lower scabbard ring brass is 16 inches. (( the blade measures 30 1-2, the overall length with scabbard is 36 1-2)).
The officer looks fairly tall, but maybe only slim, as men of that era on average were at least 4-5 inches shorter than today. His frock is regulation Yankee army, while the lower photo is regulation VRC.
Can you make out any bars on his shoulder stripes, or is this positively a second lieutenant?