Each game has a limit designated by the game developers.
Each cloud save call from the API has 2 configurable limits, but this applies only to the code in the game if it chooses to do cloud saving itself instead of letting Steam do it after the game closes so these don't apply.

This game has a limit of about 1GB for total save storage.

If you go here and login (it's steam's official site, it's fine) you can look at your entire steam cloud and download whatever files you want.
https://store.steampowered.com/account/remotestorage
You can't delete any of them from the site, but you can view and download the files.

Anyway, if your save file is too big for steam, you can turn off the steam cloud for the game by right clicking on the game, selecting Properties, and then unchecking the steam cloud sync option. This will make it ignore Valheim saves.