envelope question

Hi,
I have a question about envelopes.
I am ordering:
a 6x8 invitation
a 3.5x5 response card
a 3.5x5 enclosure card
all on 300gsm somerset paper
direction card
hotel info
on regular card stock

What size envelopes should I order if I want double envelopes?
Will a 6.5 x 8.5' serve as an inner envelope and a 6.6' × 9' as the outer? Or should i order a 7.75' × 11.5 as the outer?

Also what size is a A8 envelope?

Thank you for your help!!