I don’t think Apple has ever made their own phone screens. They are a not a manufacturing company.

The limitation was the size of the human hand. Jobs wanted it designed to be single hand navigable. This was widely broadcast and know since the introduction of the first iPhone, and criticized once android phones got larger.

The last form factor Jobs oversaw was the 5. The screen did change so a preexisting contract wasn’t constraining the size. The screen got slightly taller but was kept the same width. It had the smallest screen of major phones released that same year, and was still smaller than the flagship android phones of the previous year.

The first form factor that jobs did not oversee was the larger 6 and even larger 6plus.