If the Confederacy leaves peacefully, then they'd have the original 7 states. This would require a CSA President that does not fire on Ft Sumter and is willing to offer trade concessions in return for foreign recognition. It also requires a US president who does not feel he has a duty to keep the Union united. This will require different Presidents for both sides than in OTL.
The other option is outlasting the Union. This would need a CSA President who cares more about his general's competence than their friendship, merit more than seniority, and is willing to offer trade concessions in return for foreign recognition. That requires a different CSA President than in OTL. It also requires a peace candidate to win the 1864 US elections. There weren't any in OTL, though President McClellan might bungle things bad enough the Union eventually decides the war cannot be won.
Best case in peace by exhaustion is the CSA loses West Virginia and Arkansas. More likely, they also lose Tennessee and Lousiana. Smallest likely surviving Confederacy consists of SE Mississippi (1/2 or less of the prewar state), Florida, Alabama (minus the northern part), Georgia (minus the northern part) South Carolina, North Carolina (minus the Alleghenies), and Viriginia (minus the Alleghenies, the pro-Union southwest, and the Union occupied northern part).