That's a good question and one I ask myself often. You can download servo charts that will help you some. There are so many options out there it's hard to say, "get this if that" and get this if it has this, etc.
I have built about 7 planes now from kits....all with wingspans from about 48" to 72"....all are "beginner type" planes that aren't going to be flown real hard. I put the same servos in all of them (Futaba S3004 on the throttle and S3010 everywhere else)
They have all flown great. But I ask myself....did I need S3010? I have no idea. Hinges and hardware I always err on the safe side. Usually the hardware comes with the kit and I have never had a hinge fail or any of the hardware fail for that matter.
I'm sure there are a lot of guys out there that have more knowledge to answer your question than I am but its food for thought.