Bolt Length For Top Cap
This expander does not come with a top cap or top cap bolt. What length bolt is required with the top cap to pre load the headset. Looking at the pictures of the expander the top cap bolt needs to be long enough to reach into the expander to catch the threads.
Any length bolt would do - the bolt meets the thread of the bung about 5-10mm down, so as long as your bolt is at least that long (obviously it will be) then you'll be fine. It's hollow, so there's no maximum length for the bolt.