How much cold storage do I actually get? Is it a fixed amount?

Paid plans bundle a cold storage allowance alongside your hot allowance — on the 1 TB plan, for example, you get 1 TB of hot and 1 TB of cold. The exact figures for your account show up on the Subscription page, which lists hot and cold allowances separately. It's a decent chunk of free archive space that most photographers underuse, honestly — if you've got old weddings sitting on external drives, that's what the cold tier is for.