I do not see how a public domain can be 600,000. The Gemara Shabat (in the chapters about carrying in a public domain) talks like it is a common occurrence. [I am thinking mainly about the chapters about what a man or woman can wear in a public domain.] The reason this number 600,000 seems unreasonable to me is that no ancient cities had anywhere near these numbers, Even Rome had 324,000 according to the census taken in 152 BC [as brought in Livy in the collected fragments. book 48.]
The Gra brings a proof to the Rambam,Ramban, Rashba that a public domain does not need 600,000. the gemara in Eruvin 6b: Ula said if not for the fact that they close their walls and gates at night, Mehuza (a city in Bavel) would be a public domain. And Ula also said "There is no city in bavel that has ''uchlusa'' 600,000." Besides that, I have noted in this blog before that no ancient cities had anything even close to 600,000.
There is however such a thing as an eruv, but that can only help in a Carmlit, not an actual public domain. [A Carmlit is sort of like a private domain in so far as it is permitted from the Torah to carry there. But it is unlike a private domain in so far as carrying to and from there into a public domain is not obligated a sin offering.]
