Series 3 questions

1. No one knows except Apple. That said the 5s was at the bottom of the supported devices with iOS 11. To counter this, Apple have muted that iOS 12 will focus on bug fixes and stability, so...

2. Absolutely. I do this all the time when driving. Not just Spotify but other media apps as well such as Plex.

 

For number 3, get the 42mm, much efficient when reading on it, whether it's letters or numbers. I have slim wrist also, and got the 42mm.

 

Reference your questions for #1 and #3:

1.) When watchOS 5 launches, I would be inclined to believe that the iPhone 5s will no longer be supported for the Apple Watch. It Will likely move up to par with iPhone 6 and upwards.

2.) Even if you opt for the 42 mm, you just want to make sure the band is secure enough for direct contact on your wrist to have an accurate reading. Every band will fit differently in adjustment, but I think the best bands for the most adjustable fitting would be the sports bands.

 

Anything would be 100% speculation, but the 5S does have a 64-bit CPU, and it is also a quite fast phone still, so next iOS version is absolutely at least a possibility. And if the 5S gets iOS 12, then it would also get next apple watch compatibility without a doubt, because it's not in Apple's interest to fraction their market and create confusion for its users. If one watch works, they'll most likely all work.

After all, Apple does like to keep its ecosystem as coherent as possible with as many users as possible on the latest version of system software, so they probably don't want to artificially cut anyone out because then people like you couldn't give them more money by buying watches and whatnot.