I am doing a research for encrypting data, I found many research doing multiple encryption using DES and RSA ? but in symmetric cryptography the best algorithm is AES so I need to do multiple cryptography for data using both algorithms ? Is there any rules to follow this way specially in the key length ? any recommendation ? Thanks in advanced...
Yes, nowadays DES is not really secure anymore.
Yes, for usual applications, AES properties allow it to be be used as drop-in (as long as the different block and key size is kept in mind).
RSA has a maximum data length, but for usual RSA key sizes and padding modes (eg. 4096 OAEP), encrypting AES keys is always possible. See What is the maximum size of the plaintext message for RSA OAEP? for more details how to calculate the maximum length.