Minimum brake rotor sizes
Since post mount became the norm, most long travel/gravity forks have a minimum rotor size based on what post mount length standard they're built with. Here is how Fox communicates it on the product page for their 40 series forks for example:



So putting the caliper directly on the post without any adapter sets it up for a 203, and the fork can't take anything smaller. If you had a fork with 180mm post mounts and you wanted to run a 200mm rotor, you would use a +20mm adapter, and you would not be able to run 160. If it's not specified anywhere, which is often the case, usually that means 160 front, 160 rear for mountain and hybrid/urban and 160 front, 140 rear for road.

Sometimes manufacturers make it difficult to track down the number. SRAM/Rockshox only sticks it in their giant tech spec pdfs as far as I know.

This is the answer as it applies to current forks. In the past there were more forks that needed model-specific adapters, and various IS-only calipers based on specific rotor sizes. So in the scheme of things, there are also all sorts of older forks and brakes where the minimum or maximum rotor size was limited in those ways. In the more common case of a fork with standard IS mounts, the minimum is always 160.