I am a marginal COS (correct term per WN: Customer of Size). You will be challenged. I have been on a few occasions, but I am overweight, I have purchased the seat and I have a standing joke that diffuses the situation in my favor. "If I am not a COS I am just a double cheeseburger and an order of fries from being one." Its gets a chuckle and a gets me a pass.

The gauge is whether the armrests go a down on the 17" wide seats. For me they will go down snugly but not all of the way. They rest on my legs. Like many men, the extra weight I have is not between the seat arms, it is a bit higher, and you would be uncomfortable sitting next to me unless we really liked each other.

Flying as a COS has its drawbacks. You have to call and buy your ticket unless you are your own companion as I am. I "buy" my extra seat when I get my companion booking. You can not check in on line. You can not bypass the ticketing line as you must get your "Seat Reserved" card from a full service station. This is the most likely place you will be challenged, not at the gate.

If they decide that you have violated the T&C they can revoke your ticket and possibly take away your RR account, although either of these would be unlikely.

Also, they do not have to let you and your wife board together. The COS must preboard, but it is the OA's discretion if the companion preboards. Back when we had to get blue sleeves this was not a problem as an assistant can board with blue sleeve passengers. The Seat Reserved document is now used to preboard and does not get blue sleeve privileges. You would board first, but you would need to secure the seat for wife. That means you have to hold the whole row. That may not work out so well for you.