Hi Avinash
I wouldn't say there are any standard sized brochures, most people tend to us A4 or A5 when designing a booklet or a magazine as thats what they may have a knowledge of and might not want to deviate to unusual spec'd sizes.
Then again there might be no need to.
If you want to create a bifold brochure best talk to a printer who you deal with and ask, i know printers i deal with are happy to answer my questions and suggest ways to do things, as that normally means they get work! :lol: