I'm not sure I would use size as the indicator of which one to keep because it comes down to personality more than anything. Some roosters will always be nice, friendly and gentle, while others will try to dominate you - regardless of size. I live in the city so cannot keep roosters. That said, I ended up with four (straight run chicks) and just processed three of them last Saturday. The fourth picked up crowing now that he is the "head roo" but so far does it just a few times around 7am and I just wait until he's done to open the coop and let them out. I doubt if any of the neighbors are even aware of him, since we can't hear him from in our house a short distance away, unless we open the back door. I wish I could keep a rooster because a good rooster will help take care of the flock - give a warning when a hawk is spotted, try to defend against some other predators, find treats for the girls and so on. I agree with you that two roosters out of six is too many but I would wait to decide which one to keep, and see when they are old enough to start acting like roosters, which one is the better one to keep. For culling, you could just post an ad on Craigslist to try to rehome him, but be aware that more than likely the person who takes him off your hands will do what you don't want to - process and eat. Each person has to make this decision for themselves. For me it came down to: I know they led good lives (pampered really) and were happy - much more so than the chickens available for sale at the grocery store. We processed in a very humane way. The "investment" in their food over the last three months provided us with much appreciated food for our table.
Unfortunately, around 50% of the chicks hatched are roosters and 9/10 of them are not needed in this world, so difficult decisions need to be made. Keep in mind that if you do decide to breed, you will face this decision again in the future as half of the chicks you hatched will also be roosters. If this is something you don't really want to deal with, you might consider getting rid of both your boys and replace them in the spring with a couple more pullets.