If they follow the iPad size logic on Kuos Chart, it would hint at 14“ for the MBP. One small consumer size and two larger pro sizes. Would make the portfolio more consistent. Same is true for the phones, although the sizing does not work out there.
If they do, but that's my point - that's nothing but speculation. The chart is Ming Chi Kuo's prediction for 2020 MacBook releases, and as yet it's the only thing we have to go on that is more than just speculation. Until more evidence comes to light, or Kuo releases a new note correcting himself, this is what we have to expect to be released over the next 12 months. Perhaps Kuo is wrong here, perhaps he was right at the time of release but Apple change their plans, perhaps his next note will reference a 14", but until then it's pure speculation.
To pick up on your point about having size options only at one price tier, in the case of the MacBooks, I actually think it would make sense for the size option to be in the middle, and this also fits with Kuo's prediction:
16" Pro - biggest, most powerful, most expensive. There's little room for a complimenting 14" here as they had trouble with even the 15" size with this class of machine and had to make it bigger.
13 and 15 inch Pros - would cover the broadest base of customers. Pretty powerful but not as break the bank expensive as the 16". As the most flexible machines it makes sense to have the size option here. At present there are two near identical 13" models, yet one is considerably more expensive. They could dump the more expensive 13" and put the 15" back in at its price point with roughly equivalent specs.
13" Air - small and focused on portability, battery life and is the cheapest/ entry level model.
