Re the most people wanting large, is that just because they have just agreed to the up sell?
When I order I forget to say the size, I always just say yes to the large.
Nope; if people don't want a large they are very clear about it.
Although I will remember in future orders to ask for the size as did not realise you couldn't start the order until you had the size.
We appreciate that 
You could say "what size", its shorter than "is that a large"- so does not take any longer and no less polite than is that a large.
No; it's too abrupt.
It is an up sell- ask your area manager the difference between profit on a medium and large- they will know.
I'm fine, thanks.
I do appreciate you are not saying it thinking its an up sell, that's the point, and why McD's is such a well oiled machine.
No need to be patronising.
I think its a bit disingenuous to say a happy meal is the small option, can I get a big mac/wrap of the day/ etc etc as a small?
You could buy a Big Mac/Wrap of the Day etc. and purchase small fries and a small drink, of course, but it will be more expensive that if you were buying medium.