BJ,

It is said that at least 28.5 feet length room is required for the
control room cuz I thought the wave length of 20hz is 56.6 feet

That is a myth. If it were true, you wouldn't be able to hear anything below
a few hundred Hz in a car! In fact, even a small room like yours can
reproduce very low frequencies. However, the *flatness* of the low end
response is undoubtedly way off.

I am using Mackie Hr824 monitor speaker in my room but my room is really
small. most long length is just 12feet

Your speakers are fine, but your room is surely in need of bass traps and
other acoustic treatment. For the complete story see the Acoustics FAQ,
second in the list on my Articles page:

www.ethanwiner.com/articles.html

--Ethan