For riding in Norway, practically any bike size will do. Actually, a too big one will probably be more restrictive than a smaller one.
Of course you’ll have fun on your Triumph Rocket III. Even – to a certain degree – on your Boss Hoss, if that’s what you have, even though you might struggle in the tightest bends of the Lysebotn serpentines. I have toured extensively on several bikes, mainly Guzzis from 750 ccm upwards to my California 1400. I must admit, though, that in the last year or two I have toured more and more on my small capacity Yamaha WR250R. It’s more than adequate for the low speed, narrow Norwegian back roads, where you should spend most of your time anyway, while still being able to do highways (which are a max of 110 km/h anyway) without being stressed. Why, my daughter and I have even toured on a couple of pizza delivery mopeds. We didn’t venture too far on those, but it is still doable.
There are not that many long, boring highways in Norway. Except for the southern parts of the E6 and E18, and a few stretches around Trondheim, Stavanger and Bergen, the roads are usually quite small enough to be capable of catering for 125 ccm bikes too. Which means that if you have a learner youngster at home who wants to tag along, or you are a learner yourself: Do not that let it prevent you from coming over.
It’s the mindset that counts – not the tool, right?