I am a CC and recently a "pax of size" on a Monarch airlines A300 for a reasonably short trip....2.30 mins approx. My "size" is 12-14 depending on the clothing manufacturer.
Knowing that the seating on this plane is to the max capacity (have a pilot friend in Monarch) and the legroom is tight I duly paid the excess for some legroom. There were 3 of us travelling and we were seated at the emergency exit towards the rear.....legroom therefore guarenteed. Width of seat narower because of tray table stowed within it. Not a problem for my party.
What concerned me that directly to my right (on the emergency exit row) was 2 pax of so much "size" they needed an extention seatbelt. Also when the CC responsible for that exit sat down for take off (her modified seat was directly next to them) she had to manouver herself into it to avoid them.....what would she have done in an evacuation situation if they were incapacitated?
In my opinion (and it is just that) pax of excessive size i.e needing an extention belt should
never be allowed to sit at the emergency exits legroom or not. That also is the policy of some airlines.
Obesity is an increasing problem in the UK and
all airlines need to wake up the that fact and maybe have a look at what their USA counterparts do to deal with this...they are somewhat more proactive than we are. It can impact on pax comfort and after safety the CC are there to try and ensure that within the resources they have.
Seated next to a pax of size which encroaches onto your personal space then the CC will attempt to deal with this within the resources and seat allocations at the time hopefully with diplomacy on all sides.
Do it after the door closes expect a different approach from the CC
We have safety priorities at that point.
Bealzebub summed up the position on this and I totally respect him/her for doing that.