Has anyone encountered this on v42.0 and if so, was there a resolution? I am overriding the "New" Case action with a Lightning component.
Strangely enough, when attempting to deploy to an org that org on API v41.0, the message is instead: objects/Case.object -- Error: Large is not a supported form factor.
In the Case.object metadata, here is the relevant action override section (you can see formFactor is indeed set to "large":
<actionOverrides>
<actionName>NewCase</actionName>
<content>CreateCaseCmp</content>
<formFactor>Large</formFactor>
<skipRecordTypeSelect>false</skipRecordTypeSelect>
<type>LightningComponent</type>
</actionOverrides>
However, a value of "large" seems to be what the documentation for the actionOverride metadata requires for a lightning component override -
the Large value represents the Lightning Experience desktop environment and is only valid for the flexipage and lightningcomponent types.