in most countries, no document will help you. You'll have to present yourself to the nice cabin crew as soon as possible and they'll do their best to accommodate you. Although nice check-in staff *might* be able to upgrade you too depending on the airline. In Canada, now that oversize pax (wide, not so much tall) can have two seats for the price of one, you might get some luck there, but it needed a court to decide this. Where are you? Which airlines are we talking about here?