This is according to internal Apple emails that were revealed during the Apple VS Epic trial, where in 2007, an email from Apple’s late co-founder Steve Jobs made reference to a 15-inch MacBook Air. The MacBook Air officially made its debut in January 2008, but clearly it seems that the 15-inch model did not make the cut.
It is possible that Apple later decided that a 15-inch MacBook Air would encroach on the space of the 15-inch MacBook Pro, or that a 15-inch MacBook Air would make the laptop too big and heavy and negate the point of owning a MacBook Air. Whatever the reason is, it seems that the 15-inch model was ultimately shelved, although there are some rumors that suggest that it could be making a comeback.
Apple has experimented with various laptop sizes over the years, like with the 12-inch MacBook that was later discontinued. They also eventually released a 16-inch MacBook Pro after doing away with the 17-inch model, and word on the street has it that a 14-inch MacBook Pro could be in the works.