If you want to go to war just go to war. Don't think about it too much. Of course you should go for someone of your size.
If not grab an ally but don't grab 2~3 all at once. The more you grab, the more your enemies will call for as well.
Just don't try to win every war. You can't afford to wipe realms out as you can't support large realms that easily anymore.
Reasons for war? Pfft who cares about those. When you feel like going to war, you just go to war. That is good enough reason. Don't let silly people and their fancy reasons to stop you
