Against someone roughly my size and strength, I typically hold up really well. Mainly because I spent a lot of my time fighting guys bigger than me, either in sheer girth or in strength, and I was always able to hold my own. Plus I learned how to take a beating damn well early on, so that helped out a lot, especially against guys bigger than me, I can hit hard, harder than most people expect, so often it only takes a couple punches against someone who's not quite expecting it and they're down. I've got a bunch of buddies who went in to the military after high school and I still can hold my own, if not beat, them in straight fights.
Bit of a story on that note, I was walking across the intramural fields on campus in the middle of the night and a few guys tried to 'sneak' up on me. I can only guess they were trying to mug me or some such (it had happened to a friend before then, so that's what I assumed). I just kept walking, occasionally checking over my shoulder, and the second they started running (they started shouting at me simultaneously, which is how I knew they weren't going to be asking for directions) I took off, but not too fast. One of them outpaced the two others a bit, and I ended up stopping faster than he expected. He ran in to me (tried to throw a punch as he did), I grabbed him and threw him to the ground pretty easily. From there it was fairly simple to just put my shoe on his face and hold it there while telling the other two to back off or I'd break his nose (and likely his jaw) in. That solved the problem pretty quick - they just backed off, I stepped on the guy's stomach (just walking over him, not with too much real force) and walked away. Apparently they weren't ready for someone to actually fight back.
As for the second question, it depends. I won't start a fight unless its with a friend, but there are occasions I'll force people into an ultimatum. For example, a guy was hitting on my girlfriend a year or so ago, and I ended up telling him (after a couple hints, and not-hints, that he should stop) that he needed to either back off or hit me. He ended up backing off, sadly enough... I was really looking forward to kicking his ass. Ever encountered one of those guys who you just feel would be a lot less annoying/douchey/cocky if they had gotten their butts kicked once or twice as a kid? He's one of them.
Anyway, I'll get in to a fight over anything halfway serious, but I'm careful about who I'm fighting with more than the reason. Someone I know I can trust is a lot easier to fight with because we'll just shake hands afterwards and go back to being buddies. Someone I'm not sure I can trust I won't throw the first punch, but I will make them do it if it's serious enough.