Torso body armour sizing

I am looking online for torso body armour and many sizes are 36-38" then 40-42". I am interested in the Brand X X suit mostly as it looks quite good for the price and the reviews are positive. Nuke Proof sizing is 39-42" but too pricey for me at the moment. I have a 39" (relaxed) chest so am thinking I should probably get the 40-42" size as I'm not going to get any smaller and I can imagine I'll be gasping for air in a 38" suit. This is what I'm thinking, anyone else with any ideas or thoughts on this?
Thanks.