Thank you for sharing the template - it really does help put the word out :).
As far as a level size limit, the template has no limits, so you are only constrained by Unreal Engine's map size. If you want a really big map you will have to look at world composition and world origin re-basing. I haven't tested a massive level yet, but I don't see any technical limitations with the template itself.
I've seen other space sim projects with maps that are thousands of Kms big and they work fine as long as you use the proper setup.
Hope this helps!