How do I choose blind sizes?
You can try to encrypt your sensitive data with a lightweight cipher, like Prince. You encrypt your Prince key with a HE encryption scheme like BGV (HElib implements it) and give the cloud two things: Enc_HE(Key_Prince), Enc_PRINCE(Data). The cloud is able to decrypt homomorphically Enc_PRINCE(Data) using Enc_HE(Key_Prince) and obtain only Enc_HE(Data). The cloud can now perform unlimited computations on encrypted data. Your Key_Prince will not be found. You can find more about this scenario in: Toward Practical Homomorphic Evaluation of Block Ciphers Using Prince and Homomorphic Evaluation of the AES Circuit. HElib can be hard to use, do you might instead try a more friendly library, SEAL. But if you want to do only additions or only multiplications, there are more practicals schemes than fully homomorphic ones.