Protestant league started out quite small with a few HRE minors. I was minding my own business as Prussia, waiting for the chance to join the Protestants so I could strengthen them massively and challenge the Catholics (couldn't join at first because of truce with Emperor, then was busy with a war in Eastern Europe), then the war broke out - I didn't even notice how almost whole of Europe joined leagues, with Burgundy, for example, a Catholic nation, going to Protestants. I wasn't in any league, but Burgundy called me to arms and I could join, which is nice. The forces were balanced with each side having enormous armies, but the Protestants ended up attaining a huge victory (we did lose a few minors who peaced out), and some why I, as Prussia, was made the Emperor after the war ended with a Protestant victory (I was protestant obviously), which was totally unexpected as I was not intending to be involved in HRE that much.
What I found frustrating was that Norway somehow became Protestant League leader (probably because they were first to convert, dunno) and were thus the war leader for the powerful Protestant side. Some weak minor was dictating the war terms of a side consisting of some of Europe's most powerful great powers, meh. Saxony was war leader on the other side, lol, but they were the Emperor, so all good there (Emperor apparently is automatically leader of their respective league - I was made leader of Protestants after the war ended & I became Emperor).
It was fun, a really huge and balanced war between pretty much of all of Europe. But it can also turn one sided, so I guess I got lucky that the randomness factor gave me a pretty nice first impression of the religious wars. Catholics never came back from the massive defeat and Protestant eventually turned into the official faith of HRE.