Diffie-Hellman key size
Diffie-Hellman key exchange protocol has two key sizes, the discrete log group size and the discrete log key size. More specifically, the discrete log group size is the length of the modulo \(p\), while the discrete log key size is the length of the exponents, \(a\) or \(b\), chosen by Alice or Bob.
Currently in 2018, the recommended value of the two sizes is 256-bit for key size and 2048-bit (always the same as the recommended key length of factoring based crypto-system, RSA for instance) for group size. (The latest recommended key length can be found in KeyLength)