I will assume you're running Windows so I'd recommend the usual: Veracrypt
You can't really use it to encrypt folders, but you can use ti to make encrypted containers - these are files that contain encrypted content. You can mount them like you would any usual drive and they appear in explorer under a given drive letter i.e. G:\ then copy anything of value inside and it gets encrypted transparently. These containers can be set to grow dynamically, so there's no fear of running out of space unless the disk also runs out of space. Veracrypt can also encrypt entire drives as well as the system partition you are running your operating system on.
Its code has been audited. While some bugs have been found they have already been patched. One thing to note is that there are no known backdoors in this program which can be a good or a bad thing, depending on how you view the situation. On the good side it means that the system is secure and the encrypted contents will be safe from the prying hands (assuming you won't get rubber-hosed). The bad side is that should you forget the passphrase to the container or device then the contents are gone forever.