Really depends on the type of programming you do. For many cases, any of those three will be more than enough.
If you're doing a lot of data crunching, data science, or other computation - then you'll need more CPU/GPU/etc, which takes you down the 15" route.
I opted for a 13" Macbook Pro as I travel a lot, work away from my desk a lot and generally have my computer with me anyway. If I was desk-bound I'd consider at 15", although I'd also consider an iMac/cheap laptop as an alternative combo.
I'm not a huge fan of the new Touchbar series. Actually think if Microsoft keeps it up I'll consider a Surface next round.
