Oh, and nightmare if you haven't gone back to Who Framed Watcher Wabbit addon page since your last post I added the code fragment that is needed for it to work with nUI. I'll leave the choice up to you to use it or not though
Adding more quests with the settings you have shouldn't make it appear as the scrollbar is based on the size difference of height and scroll. If you track more than the space available it just doesn't show the extra quests. But then you can always increased those two values making sure to keep that same less than/more than difference there.
If you find you need more height space that can be arranged. I thought 800 height max and 600 width max would be plenty for most screens allowing extra room for those people with bigger monitors.