You can't currently trigger events/interactions when you scroll so you can't make this perfectly yet.
One way you could prototype this is to create different arboards, which one showing the shrunken header. It won't happen on scroll, but you could link to that artboard so someone can see what it looks like when it's shrunken.
Another thing you "might" be able to do (depending on your design) is to hide the shrunken fixed header behind the current header. If the shrunken header position is fixed, it could slide out from behind the larger one when you scroll. Again it's not perfect, but could some something close enough using the features we currently have.