Upload size is capped at 50 MB for free accounts and theoretically unlimited for paid accounts as long as it doesn't exceed their storage capacity.
Why "theoretically" unlimited for paid accounts?
Due to strong encryption, file sizes greatly increase upon upload. There is no easy way to detect how large a file will be before it is encrypted.
Encryption happens in your device/browser so that we can't see the file's contents. And since we have to encrypt on your device, this operation uses your device's resources (like memory & processor). This means that files can be as large as your devices can handle. The greater your device's specs and performance, the greater the size can be.