Registering birth abroad - which size prepaid Special Delivery envelope?

We have an appointment to register the birth abroad/apply for first passport of my daughter on Friday - I know you need to bring a prepaid special delivery envelope, but which size is required?  I've bought a smaller (half size) one for up to 500g but now am thinking I should have gotten the larger one.  does anyone know what size is recommended and what exactly is returned in the envelope?  I have no recollection of what we did for my son a few years ago!

thanks in advance.

Only the passport will be sent in the envelope. So if your passport fits, it should be fine. You'll get the Certificate the same day and the SS Card will show up about 2 months later. We got DD's passport in 7 days.

ah, fab - thanks so much!  I wasn't sure if documents would be returned in it as well - the smaller size should be fine then.

The small one works fine.  And if you are registering more than one child you still only have to have one envelope.

thanks, Mindy - that's really good to know!