What's a good safe max size for the Archive folders?

I have a lot of old emails from the last 20 years. I’m using auto archiving to keep my inbox from getting too large, since in the past that has caused instability. But what about the size of the archive folders - Inbox + Sent?

Right now I have my really old mail folders as subfolders beneath the archive folders. I have about 800,000 old emails. As of now they are in folders of about 10,000 each all as subfolders beneath my archive folder.

Here’s my folder structure:

my email address
Inbox Sent
10 subfolders 5 subfolders

There’s no doubt v8 is much more stable than v7 for me, but I don’t want to push it. It’d be more convenient to have all the old emails in just 1 subfolder, or better yet in the archive/inbox folder, but I’ve been leery of that. So any advice would be appreciated.