Sizing obviously depends on your body shape and how you prefer your clothes to fit. Someone can say they prefer a tight fit, but have a larger upper body. So while they may be the same height as you and wear the same size, that same shirt will fit loose on you.
Your best bet would be to get to the nearest Jiberish store, but if that's too far of a drive, then go to the mall and try on dress shirts that have specific sizing and figure out your measurements. Or get sized by a tailor, then you'll have no doubts.
Then of course, you can just switch the shirt for a different size after your order it, Jiberish has excellent customer service.