From RC Helicopter Wiki
Model helicopters come in a variety of different sizes. Sizing terminology is rather ad-hoc, but aircraft can be generally divided into categories based on their weight and main rotor diameter.
The names of the sizes historically come from the displacement volume of the nitro engine (with a "thirty size" helicopter having an engine displacement of 0.30 cubic inches), or the size of the equivalent brushed motor required, but this convention is not universal.
- Sub-micro (<100g, <20cm rotor diameter)
- Extremely small helicopters, often coaxial. Low total energy makes them suitable for flying indoors.
- Micro (<500g, <50cm rotor diameter)
- Small helicopters such as 350-sized electric helicopters. Mostly fixed pitch helicopters, a few collective pitch helicopters
- Miniature (<1kg, <80cm rotor diameter)
- 400 - 450 sized electric collective pitch helicopters, can be flown inside in large open spaces with care.
- Small (<2kg, <100cm rotor diameter)
- 500 sized electric helicopters, very small nitro machines.
- Normal (<4kg, <150cm rotor diameter)
- 30 and 50 sized nitro helicopters, 600-sized electric helicopters.
- Large (<6kg, <200cm rotor diameter)
- 60 and 90 sized nitro helicopters, 700-sized electric helicopters.
- And beyond...
- Above this size are very large helicopters, often powered by gas turbines or 2-stroke petrol engines.
 See also
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