Holy smoke. The 2018 15's is the Lexus grille of trackpads.
15 might be more suitable if you're an older guy.
I have a 21" external monitor that works fine for me too, doesn't mean I want it on my MBP. You happen to like it and there's nothing wrong with that. Many actually do.*shrug* it's about the same size as Magic Trackpad 2 -- which I've been using for quite a while -- so for *this* older guy it works just fine.
Way too much of a personal choice to decide without getting hands on. Go to an Apple Store. They usually have the 13 and 15 displayed side by side, or will set that up for you in the business area.
I can tell you I walked into the store dead set on getting a 13". I walked out with a 15".
Partly because the 15" was not as much bigger. But, also because by the time you get to 512GB SSD and 16 GB memory on the 13" you are getting close to the price of the 15" pre-configured with these sizes.
Programming with two 80-character-wide editor windows is cramped on the 13" but fine on the 15".
(Personally I won't use any Scaled (downsampled) mode because the image quality is worse than native and it saps performance a bit)
Well if we're getting picky, I'd call that "default", not native."native" depends on the model (e.g., different on 2015 and 2016, even though the screen itself has the same resolution).
as the saying goes, it's not about the size, it's about how you use It
for me personally even tho the 13" is smaller I find myself working much more efficiently on that smaller screen as everything is more focused and less spread out.
Until you have an IDE and other applications that you need to use in tandem. That is literally impossible on 13", trust me, I tried many times , you end up just having to switch apps rather than have them side by side.