how much will your aircraft weigh? Is it for fixed wing, or rotor craft? How many props do you plan to spin?
These are important starting points. You should start with a weight range, then look at motor/prop pairing.
Once the general weight range is known, and the number of props are decided, only then do you want to look at specific propulsion solutions.
And when you do, you should look at finding the most efficient motor/prop combination, delivering the correct thrust for energy cost at the required RPMs... this determines the blade length, not the geometry of your aircraft (except that if you know you are building something small, you obviously cannot have props that are too large to fit)
And *after* that, you look at the distance between props to see if it is workable for you. NASA has some good (old) documents describing the efficiency losses when your props are too close tip-to-tip...
As a good rule of thumb, you'll have much better efficiency if you place one radius between the tips of your props. If your prop is 10", then having 5" between the closest point between two props is a good starting point. Everything in multi-rotors is a compromise. Reducing the distance between props means less weight (less structure), more maneuverability (props move through less of an arc when rotating the airframe around an axis), a smaller airframe (easier to transport), but it also means that the air turbulence makes the props work harder (increased resistance) to degraded thrust (disturbed air) which means slightly less weight carrying capacity and lower flight times (more energy use)
It is all a trade off.... but here is a secret of efficient design: everything starts with the weight budget. Chose your motor/prop/RPM based on the target weight of your aircraft, and you can design an efficient system.
ok great. lets assume i need a hexacopter, bale to lift as pay load 1kg.. what would you recomend?
I wouldn't. I would recommend a quad ;-) Designed to lift 1kg, plus the aircraft weight, plus battery weight, times 1.15 - 1.5 as the window of thrust required, divided by four. Then account for 20% efficiency loss (i.e., add to the value, and you have a good thrust range. Find a prop that will deliver that thrust at peek efficiency (flybrushless.com) and find a motor that will drive the prop at that RPM with 80% efficiency (also flybrushless.com)
Or.... ask yourself if you really need 1kg, or if 500-700g will do. Then ask Jani at jDrones which platform to buy/build for the new lift requirements....
it makes sense