While the images do show Silver Age bags and boards, the language doesn't specify. CGC could have sent larger sizes, but it's become obvious that they are focused on serving the modern market. I don't know if you were just redeeming your membership kit, but the majority of people paying CGC for shipping materials are probably going to be unfamiliar with shipping in general. I assume that would mean young people and I assume young people are primarily interested in moderns.
There are ways you could get individual silver/regular sized bags and boards, but I'm sure the consensus is that if you're in deep enough to be submitting comics to CGC, then you're probably in possession of silver/regular bags and boards, if only by accident.
Also, you might consider that if your book is good enough to slab, then it's good enough to send inside mylar with a thick backing board.
What are those comics sitting in now?