What is the ruling for Warhammer base size when GW has not produced an official model?
There's no hard and fast rule for it.  I would suggest using a base size most appropriate for the size of the model.  So if the model is Terminator sized, use a 40mm, if Dreadnought sized, use a 60mm.  So convert your model and use what makes sense.  

This requires a bit of good judgement and common sense.  If you made a reasonable selection most people will be OK with it in a friendly game.  Though you should always check with your opponent before using something non-standard.  If you plan on using your converted models in a tournament, contact the tournament organizer before hand to get a ruling.

As for units that used to be released with different base sizes, you are still free to use the base they came with, but I would recommend re-basing with the most up-to-date sizes.  This will keep things consistent if you ever need to mix and match new and old models.