The Potsdam Giants seemed like a pretty cushy job for those times. According to the article, they never saw battle. I'm doubting the 8'6" tall fella though. Anyone that tall wouldn't be worth a darn in a fight. He'd probably have a difficult time just being able to stand on his own. Just look at people that size today. They have to use crutches or walkers. When they do stand it looks like their thin little legs are about to collapse right out from under them.
The seven footers wouldn't fare much better. Too big and slow. A smaller quicker opponent could do quite a bit of damage and be gone before the giant had a chance to react. But an army that size could/would put the fear of God into a force of smaller proportions. It was probably just to psych out the other guy. And since they never had to fight, I looks like it worked.