hm162222 asks a number of questions. Gyros are used to tell other electronic components which angle the unit it's connected to is currently at. With the quadcopter, to keep itself level the Flight Controller has to know which way it's orientated. Gyros send signals at a very fast speed to the FC telling it when it's level and if it's not, which way it's angled. Gyros can be found in some digital cameras for anti-shake recordings. Unmanned aircraft, missiles and many other gizmos and gadgets.
Brushless motors are far more robust, faster and less complicated than the brushed motors. As the name suggests there are no brushes rubbing against the armature, this eliminates sparks and wear, therefore the only friction generated is in the bearings of the motor. You could use the brushed variety in a quadcopter, in fact most of the toy copters use them and utilize small gear boxes to step the speed of the motor up. Also the brushless variety are cheaper to produce. Two bladed props are the most popular version used today. The 3 and four bladed props can be used, but aren't as efficient. A prop works by pushing air in one direction. it works best when the air in front of the blade is at rest. For optimum thrust a prop should be just one bladed, This ensures a cleaner air to bite into. Yes there are one bladed props. They were designed many years ago and worked with a counterbalance being where the 2nd blade would normally be. Some diehard hobbyists in Australia still use the on their control line planes. Such props were extremely hard to balance and fell out of favour. 3 and 4 bladed props are normally used multi prop planes where the pitch of the prop can be varied. Single prop planes and quads plus fixed pitch helicopters use the 2 blades simply because they give the best thrust to power ratio. Modellers usually only use 3 or 4 bladed props for appearance sake, although I did purchase a quadcopter a few years ago which had 3 bladed props. Because they were far too flexible and gave poor thrust I replaced them with 2 bladed props and it performs much better.
A quadcopter can be almost any size. As with anything some people always think bigger is better. In reality bigger is not only much more expensive but can get to a point where any advantage of size is lost due to disadvantages in flight characteristics. Hobbyists are always trying to push the boundaries and there's always someone to try to be bigger and better than anyone else. It's human nature I guess. There are the tiny toy variety which measure as little as 100mm from motor shaft to motor shaft and the size rises to
I think the largest I've seen was one meter and that cost over $5000 USD. Scientists are working on a monocopter about as big as a bumble bee. Then there's the biggest of them all and definitely out of reach of any hobbyist the Boeing V 22 Osprey. From prop tip to prop tip 85 feet. It can be classed as a Duocopter (2 props) when in VTOL (Vertical Take Off and Landing) mode.
Please note, some of my answers can be argued with. I only give answers from memory and prefer not to refer to search engines unless the question requires specific data. Although the answers to all your questions can be found by Googling. :-)
Happy Flying. Stu