Copter weight determines the amount of thrust you will need. How you fly and where will determine how you want to get that thrust. You can go long rotors and large, slow motors or you can go light, fast motors and short props to get thrust.
Light and fast is more aerobatic and takes less damage in a crash. Heavy and slow flies longer, is more stable and costs more to build and maintain.
You want triple the copter's weight in thrust as a general 'rule of thumb'.
I advocate light, fast motors and short rotors as they are simpler. They aren't as efficient so you won't get long flight times, but they cost less, take less to get and keep flying and weigh less so your copter will break less when you crash.
Assuming you are going to build a FT 370 kit (what I recommend for newbies), the frame limits the length of your rotors to <> 12". Motors to spin 12" rotors are HUGE and heavy and will break the Delrin on any fast landing. The smallest you can run will be <> 8" rotors. The longest you should run will be 10" - 11" rotors.
10"-11" rotors will require balancing and the motors will be <> 1000kv. You can run 9" rotors from about 1200kv and 8" rotors up to <> 1500kv depending on the motor.
I like light fast motors so I like the Suppo 1450kv motors running 8045 nylon rotors for a basic FT 370 kit for a newbie. They should generate somewhere around 2500 grams of thrust. Since the motors are light, I can keep one of these to <> 800 grams with the lipo. That gives me 800 grams of weight to 2500 grams of thrust right in the ballpark.
So now that I know rotors and motors, I need ESCs. There is math I can do here if I wanted to do it, but I am lazy and the motors already tell me max current is 12A at 3S. An 18A ESC is a good minimum.
Lipos are a balancing act (what isn't in flight?). The larger and heavier they are, the longer the copter can fly...to a point. There is a reason we don't all hook up car batteries to copters. They are too heavy. You can add so much weight to a copter via the lipo that the copter won't fly at all. The heavier the lipo the more damage it will cause in a crash.
Start light and go heavier when you can fly without smashing your copter.
A FliteTest 370 kit will weigh <> 700 grams when you are done building. The lighter you keep it, the better your motors will thrust you into the air and the less damage you will take when you crash. I find an 1800mAh 3S 35C lipo works well for learning. Move up to the 2200 later when you can fly better.
Most newbies want a big heavy copter with a 5000mAh monster that can fly for 20 minutes. Until they crash it and the lipo and heavy motors pummel the copter to dust, that is. Some will want to start with a 250 which is light and fast but very unstable also. Both extremes are harder to fly and steepen the learning curve.
If I were you, I would start with the FT Knuckle H quad or Electrohub and the Altitude Hobbies Power pack with 1450kv Suppos and 8045 rotors. It is a great place to start building and learning to fly.
