Please can anybody tell me what is the min. size of domain which fluent can solve????????

I assume you mean, how small can you make your elements? If so, any size. If your make your using gambit, there's a minimum size (0.1 micron?) you can make edges etc. So, for example if you want to make a micro sized model, you can make it in meters then scale it down in fluent to the correct size. Also be midful of what your trying to solve. Fluent solves based on continuum equations. If you go really small (~sub micron level) those equations break down.

Also, be careful with your simulation setup especially on your time step size as you want to make sure that the Courant number is "suitable" in order to avoid possibility of floating point error problem.

Thanks