So is 7zip safe compared to ZIP? I'm thinking of converting many of old zip archives to 7zip since it provides a much better ratio. For example, my save game folder for one game is 43 MB uncompressed, 32 MB zipped, and 4 MB 7zipped. But I worry about data loss, especially with older files. I don't know why.

So how safe is it compared to zip? And what's the maximum size for an archive? I'm having trouble finding info on that for some reason.

The maximum size...no idea. I never reached the limit! :D
But a simple look on the homepage offered this insight:
7ZIP is stable and from my side it never created a corrupt archive, as long as you don't use the beta versions, at least. ;)

I additionally used ECM for my 78MiB archive of Secrets of Pyramids, a tool that compresses CD images and makes them better compressible with tools like WinRAR or 7ZIP. I learned that this program can corrupt your CD image sometimes, so I would only use it with caution...

ECM corrupted the images or 7ZIP did?

neither win zip nor 7zip "corrupts" your data

Like I said, ECM did. :)