I'm answering these on the fly without checking any resources, so perhaps wait for a better answer:
1. I seem to remember from the Star Trek Star Charts that the Klingon Empire is bigger than the Romulan Star Empire, quite significantly so. The RSE seems to be 'locked in' by its Neutral Zone, whereas the Klingon Empire extends further on the other side to its border with the Federation.
3. They did have a war, referenced in the TOS episode 'Balance of Terror'.