Hello I am modeling a channel flow. I know the Reynolds number and dimensions at inlet of channel. But when I will apply symmetry I have to cut the channel into two parts. In that case how can I calculate the Inlet velocity. as Dimensions of channelwould be changed, should I take Reynolds number as constant and find velocity??? Please help.

Symmetry condiiton is to reduce the grid and there by to reduce the computational time. Here in your case (Symmetry) apply the same Velcoity and Reynolds number as those of Full channel Values. Because the velocity is constant no matter whatever may be the geometry cross section. And the Symmetry condition will take care of remaining things.