IIRC, Pocket uses Amazon's TTS service, Polly. It is quite impressive. I stopped using Pocket because their web app was updated in a way that doubled the margins, sometimes resulting in lines with just 3 words on them. But before that I used it all the time both for reading and listening.
The text styling, including margins on the pocket reader app is pretty customizable I thought.
The mobile app does offer adjustments to margins, but only if you're a premium subscriber. The web app, which I use, does not offer the ability to customize at all, and the margins are 2x as big as the default on the mobile app. They look absolutely ridiculous, and despite having contacted Pocket support about this over a year ago, they remain double the size of the mobile app (and much bigger than they used to be before the 'upgrade').
The TTS feature is pretty nifty (on Android, not sure if there's an equivalent on the web version, though firefox reader mode has TTS). With bluetooth headphones/speaker I can just have a bunch of articles read to me while I'm doing something else.
[0]: I hate random buttons for services I don't use showing up in my browser unexpectedly! I think stuff like this is part of why so many people (at least used to) avoid automatic updates. UI changes that don't immediately benefit the user.