its selected to reduce weight and size of equipments and also higher frequencies produce noise problem in some cases. so a suitable value of 400Hz was chosen.

may be you can go through these responses...

http://www.pprune.org/questions/125540-why-400hz-aircraft-power.html

Yes, I had mentioned about size reduction in my question, but what I meant was that is there any specific reason for choosing 400Hz instead of say 350Hz or 500Hz ? Is there any reason like its harmonics dont coincide with aircraft navigation or communication frequency ? I dont know.