I’d say pretty much anything will do actually. I honestly think screen size and portability matter more than anything else in this equation. What do you value? I’m very happy with my 15” MacBook Pro, but sometimes wish I also had a 12” MacBook for quickly and easily doing something. I don’t really think you need that much speed for development, even though it is of course nice to have
Wish i could have an opportunity to buy a new laptop for developing )) i'm using an old MBP 15' Late 2011 (with Catalina on board). Somethimes its tough, especially when opening Simulator, my fans going crazy, but it still do the work.
Due to Apple obsoleting my MacBook Pro I'm using Xcode on my early 2015 MacBook Air with 4 Gb ram and a 128 Gb SSD. It runs fast enough though I really need more disk space. I will buy another MacBook next year and will go for 16Gb ram and a 1 Tb SSD as it's not upgradable easily.
I have a 4k external monitor which supports USBC and I connect to that when I'm working on Apps.
(My MPB was a fast i7 machine but couldn't run Catalina so had to go. Thanks Apple)