Buying clothing online is tricky for big guys. Some men may have broader shoulders, others may have more substantial love handles. Whatever the case, half the time you're returning online purchases that just do not fit.
Though most retailers provide chest measurements, if the widest part of your upper body is your abdomen, use that as your base measurement; same for if you have broader shoulders. Most men's shirts do not taper and get smaller as you reach the bottom hem...at least not true big and tall sized shirts. You'll find in the majority of cases, it's a straight cut.