Special Delivery Envelope for passport - What size?

So I'm going to get my damaged passport replaced a week from today in London and I'm picking up a special delivery envelope at the post office this afternoon.  I just checked the Royal Mail website and they've got different sizes and I'm just curious if it matter which size I get for my passport. 

I'm not sure what else they'll be putting in there other than my passport so I'm thinking I don't need the big 1kg max weight one.

Are they not going to send it if I don't get the right one?  There aren't any stipulations on the Embassy site just this: "If using Royal Mail you will be required to furnish a self-addressed, prepared Royal Mail Special Delivery Envelope with your application."

Thanks in advance!

You can purchase one from the consular servvices receptionist. 

You can buy a pre-paid special delivery plastic envelope for up to 500 g from the post office on which you write your address for return of the passport.  This is what we have used twice by mail for extra pages and then renewal and it has been fine both times.  Keep a record of the tracking number on the envelope so that you can check when it is on its way back to you.  You can either pay for next day service before 1 pm or before 9 am service.  Before 1 pm is much cheaper and last time it was delivered at 7.30 am!!

These are the instructions from the US embassy website on renewals:-  You don't need a 1kg envelope!!  Unless you've got many passports!!  500 g was fine for one old and one new.

"To ensure the safe return of your passports, you should also enclose a pre-paid Special Delivery self-addressed return envelope that shows postage paid up to 500 grams. Inquiries regarding Special Delivery Mail should be directed to your local Post Office. As the Embassy does not keep track of Special Delivery numbers, you are advised to keep a record of your Special Delivery tracking number so you can track your passport's return to you. "

PS Get C5 size not C4.  C5 is like A5 - A4 folded in two.  C4 is like A4 flat - a bit big for passports.  More likely to get mangled ....

Thanks guys!  I got the 500g C5 one.  It was much cheaper than I thought as well!  Now just have to fill out the paperwork and get the photos and I'll be all set!

My next passport is definitely going to be staying well away from the dog.  Who knew she'd be able to open up the zipper and dig to the bottom of my purse where it was?  Thank goodness she didn't touch the page with my visa on it!!