As for sizing, I don't buy anything that doesn't have a size chart with chest measurements. That way I at least know it's an appropriate size. The "cut" of the jersey that you're referring to is a bit of a gamble. Hard to know how it will fit you. Certainly a "race" fit will be tight. And if you are overweight, then you're likely bigger in the middle than across your shoulders, and most anything may be a bit tight in the middle unless loose higher up. You obviously want one that looks loose in the middle on the models in any of the marketing photos.
I'm still heavy for a cyclist, but prefer a tight fit as I hate flapping fabric. If you are concerned about showing your rolls and "moobs", consider wearing a tighter base layer underneath. Some riders prefer sleeveless base layers, even in warm weather. And something tight can also help reduce the jiggling and be more comfortable.