I just found out that <s-button inlineSize="fill"> exists, but it is neither documented nor in @shopify/polaris-types. I really need that for nice big buttons on a pricing page. Guess I try to hack it in via a types.d.ts for now.
Yeah, I can’t figure out how to hack the types. 
Perhaps the best approach for you now would be to use // @ts-expect-error for the time being, such that you’re notified as to when it gets updated.
https://www.npmjs.com/package/@shopify/polaris-types - it looks like there hasn’t been an update to the types library for 2 months, so I’m not surprised there are some out of sync things.
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My current hack: <s-button {...{inlineSize: "fill"} as any}>
But I hoped to do some types.d.ts trickery for that, but that doesn’t seem to be possible.