There is no restriction in size by Cryptomator. Your filesystem and cloud provider provide the restrictions. So yes, your vault can possibly have a size of 300TB or more. But i guess there is other software out there which can handle such huge amount of data better. Please keep in mind that Cryptomator is no backup solution!
I’m unsure if Dropbox Smart Sync works out of the box with Cryptomator. But i found a thread where someone uses Cryptomator with OneDrive Files-On-Demand:
Bandwith is not a thing. Cryptomator is designed for usage in synced directories, so only files which are changed will actually uploaded. But if the file is still 4GB big, the whole 4GB must be transferred, since we don’t split huge files in smaller chunks due to possible inconsistency when it is not completely uploaded, etc.
Cryptomator can use at most 500MB of your RAM.
Regarding CPU usage it depends. When you perform some read/write operations into the vault of course the data must be de-/encrypted and this costs something. For example if you want to have a preview of a directory with a lots of images, you have a short peak of CPU usage until all image previews are loaded.