This is a very good question. We will update the info asap.
Obviously there is no limit for the folder size. However, when it comes to folder synchronization, it is interesting to note:
(1) When syncing files from local to remote, there is no size limit. Though in reality, the total number of files might be limited to 50000 files.
(2) When sync files from remote to local -- it requires a lot more server resources. As such, this features is turned off for free service users. For premium users, the maximum number of sub-files and sub-folders in a synchronized folder is about 3000 or 5000 files. We are still tuning the performance, so this number might change in the future.
(3) Generally speaking, you can always manually initiate a Sync operation, and there should be no limit on the number of sub-files and sub-folders.