I was looking at some bikes and was wondering if a 16 inch would be comfortable for a guy about 5'11" and around 140 pounds?

a 16" would be really uncomfortable, you should go with a large frame/ 19-20"

Maybe if you had a 26" inseam.

I think we really need to know your inseam. Also, what kind of riding will you be doing with this rig?

Depends on alot of things. I am 5' 10" and have a Marin 18.5" frame which is too big for me. I don't have any standover clearance. The top tube length feels good though. I am getting a new 17" frame with sloping top tube and it also has same length top tube. Best advice is to go to a bike shop and try out a few 16" bikes. More importantly though is the top tube length because of the design these days. I asked this question before and it is horses for courses, so many factors to take into account. Try some of the fit calculators to see what ball park top tube you would need.

I am 1.63 m tall and I ride a 16 inch frame. You are taller that I am so you need a bigger frame.

You might be ok, with a long seatpost, long(130mm) stem, and layback seatpost. My GF has a 16" Haro and I can ride it fine, but it's a little cramped.