Vanisher
On the Avengers map the starting area is four squares in and two deep, the same as the Infinity Challange rules.
The starting area has always ran the length of the map within those guide lines. So with the split starting areas they should follow the same rules.

Tell that to the Mines of Moria and SHIELD Helicarrier Below Decks S-4, that each have a funky starting area, or the Asylum, where the starting areas are in corners.

Most of the official maps follow the Infinity Challenge rules, but that would be more of a guideline. The rule is within purple boundaries, which all legal maps have. (And a lot of maps are actually slightly off of that, being 2 squares in on each side, or several that are deeper but narrower than 4 in from the sides, 2 deep.)