the ISP press release /wanted announcement does not give a height or weight:
Delphi Homicide Investigation
later FBI release says 5'6" to 5' 10"
https://www.fbi.gov/wanted/seeking-info/liberty-german/@@download.pdf
when I look at the pictures of BG, his head is down and then there is that that situation with his head/hat/hood(whatever it is) so it makes it difficult to determine where exactly his head ends IMO. Still, I would think some one who could measure the bridge ties could calculate his height, IMO.
Shadow, time of day, angle of the sun could be used to estimate height, but in this case we have to factor in that, as you say, he is not standing up straight. He seems to be slouching with his head/neck tilted down and forward, and his posture somewhat hunched or stocky. Perhaps he's hunched down on the bridge, and standing tall for police.
I'm not 100% convinced that there is an accomplice, although an accomplice hiding under the bridge would explain why the girls did not run when they got to the bottom of the hill.