An explanation of the MEGA cryptography can be found in the MEGA developer's documentation in the "Under the Hood" section.
All symmetric cryptographic operations are based on AES-128. It operates in cipher block chaining mode for the file and folder attribute blocks and in counter mode for the actual file data. Each file and each folder node uses its own randomly generated 128 bit key. File nodes use the same key for the attribute block and the file data, plus a 64 bit random counter start value and a 64 bit meta MAC to verify the file's integrity.
From this, you can deduce:
- Directory structures are encrypted through the use of encrypted nodes
- File & folder names (and other attributes) are encrypted in attribute blocks
File and folder sizes although not encrypted, will be obscured. That is, without knowledge of the file/folder names or structure, you can only guess the total size of the encrypted data.