So I checked with someone who knows crypto better than I do, and the answer is that technically the key space for an RSA key is the number of prime numbers that can be represented with less bits than the key strength, however practically it is the number of prime numbers that can be represented with one bit less than the key strength. So the key space for a 128 bit symmetric algorithm is 2^128 and the key space for a 128 bit RSA key is pretty much the number of 127 bit prime numbers.