Originally Posted by
zombietooth
You have the right to request testing it in the sizer.
True that. But there's a rule in UA Ops Specs that says that in addition to all the numbers, weights, or other dimensions of a piece of luggage, any UA employee has the right and power to decide where a bag ultimately ends up. If a flight looks like it'll have a lot if extra space, an FA or GA can allow an oversize bag in the cabin. Conversely, if space is very tight, the FA or GA can tell you that your hatbox will have to be checked. They don't make that widely known because they don't want employees getting all "power-trippy" about it, but the rule exists nonetheless
Essentially, an OpSpec becomes FAR, as in Federal Law, so best to just go along with the agent or FA; they carry the power of federal law in their words.
FAB