While I don't have it bookmarked, I actually use the wiki page on the
Territorial Evolution of Mexico as it contains maps and descriptions for every change in Mexico's history.
That is a really great link.
In a US dominated Mexico/Central America I would imagine that a lot of states are going to get combined, even states that never were combined in the first place. Depending on the circumstances, but especially if Mexico is conquered, the US is not going to accept 62 new senators into congress (or how ever many states there were). I can see Mexico being broken down relative to teh US population at the time.
For example, the US population was 31 million in 1860, the Mexican population was around 8 million. With 33 existing US states, I would say that Mexico would receive around 7-8 states and a strong chance of having an area like Sonora and Baja becoming a single territory that would be partially absorbed into Arizona and Mexico later and added as an additional state.
Anyone got some population numbers on Central America in 1860?