I think California (39 million people) should be split into four states (each with 9 or 10 million people):
1. Los Angeles (equal to L.A. County) (capital Los Angeles)
2. San Francisco (equal to bay region) (capital San Francisco)
3. South California (capital San Diego)
4. North California (capital Sacramento)
South and North California would be competitive in President and Senate races. Los Angeles and San Francisco would be irredeemable. By splitting California into four states, each state would evolve a viable two-party system. South and North California would have the Rep. v. Dem. politics. L.A. and S.F. would have the Dem. v. Far Left politics. By splitting California into two states, each part would be the same, politically, as the whole.
I am in LA County. The county line runs between two houses, making things interesting if that became a state line. The people living in OC HAVE TO drive through LA County to leave their homes.
You would also have eight U.S. Senators instead of two. At best, one or two would be Republican.