In my case (http://www.rcgroups.com/forums/showthread.php?t=1212651)
I am storage space limited and want to fly indoors. Hence, I'm planning
on a 30-40 cm frame. I believe I should use the largest diameter rotor
possible without resulting in too close rotor spacing and permitting
proper flow around the airframe. The larger the propeller diameter, the
more efficient it is in generating thrust (more thrust for least power
absorbed). For this size frame, I think its reasonable to go with 8 inch
props (APC SF 8x3.8). I think that the lower pitch is also more
efficient for hover. However, I suspect that higher pitch may result in
higher torque and better yaw control for a quad, so I may want to
experiment with that. My main objective is to have a highly maneuverable
quad capable of aerobatics. I am willing to sacrifce endurance for
this.

Next is the BLDC motor. I understand that the shorter the propeller
diameter, the faster it must spin to generate the required thrust. (I am
looking for about a 2:1 thrust to weight margin). Hence, the higher the
motor Kv should be to get more RPMs per volt. I do not understand how
to go about specifying the desired Kv. I know there are several design
computer programs out there for use on- and off-line, but I'd like to
know where I should start. Also, I'm not sure how to extrapolate the
results of those programs from the single prop to multiprop systems. I
assume the voltage remains the same, but the current and power is
proportional to the the number of motors (and that the WOT time is
divided by the number of motors). Is this right? Is there anything else I
should be concerned about on a motor's data sheet (besides the weight,
of course)?

This brings me to the ESC. I think I just need to have sufficient
capacity (amps) to withstand the expected max current draw of the motor,
right? I'd also like one that is known to be compatible with a higher
input PWM refresh rate. Anything else I should consider?

And finnaly, the battery. I know that the cell configuration (3P 1S)
gives me the voltage. How do I size the C rating? I know I should be
able to use the power required and the mAh rating of the battery to
predict flight endurance times, but I'm not exactly sure how to do that.

The other factor for all of these components is price / quality. Since I
am a beginner, I expect I will crash a few times and possibly damage
some parts. That and the fact that I'm not sure how to choose the right
elements, suggest that I should be buying cheaper parts at first.

Thanks in advance for your help.
Roy