The standard body size of a violin is 35 centimeter in length. Its lowest note is G3, which has a wave length of 175 centimeter, five times 35. 35 cm is the wave length of B5, which is a major third above G. Is this a conincidence or did anybody calculate this when designing it?
It is a coincidence, because there is no simple relation between the wavelength of sound in air, and the wavelength in the structure of the instrument (either in the wood or the strings).
The fact which is critical to the sound of a violin is the lowest vibration frequency of the air inside the body, which is about an octave higher than the lowest note. That depends mainly on the volume of the air and the cross section area of the F holes. The exact shape of the instrument (and the shape of the holes) are not so critical.
For a "half size" instrument the volume is smaller which would increase the frequency of this air resonance, but reducing the area of the F holes decreases it.
Yes, it's coincidence, mostly. Consider that 200 years ago A was maybe 415 Hz, not 440 or 444 Hz. It is "mostly" true that there's no real value to a resonant chamber more than 1/4 wave of the lowest tone desired, but there's also a lot of art bordering on black magic to create a body which produces a "pleasant" resonant strength at all wavelengths (including overtones) in the normal playing range of the instrument.
It might be interesting to do that same calculation for a few dozen other bowed string (or for that matter, plucked) instruments. However consider that half, and quarter, and even 1/8 size celli and basses are available for child players. Some tonality may be lost but overall they serve their purpose. For that matter, there are certainly "non-standard" fullsize violins & cellos & basses.
Being EXACTLY 1/5 is a coincidence (see Carl Witthoft answer). But the musical instrument size (or generally everything made to produce sound) is pretty much related to the wavelength of the sounds it produces. The laws of physics dictate that you cannot efficiently radiate sound (or any other waves, say, electromagnetic waves) using something much less than 1/4 of the wavelenght.
That's why cello and bass are bigger.