An interesting question, I think that there are no ideal dimensions as it depends on the capacity(flow rate), but the ideal shapes are rectangular( for high capacity) or circular(for low capacity).Please refer to :- http://www.thewatertreatments.com/wastewater-sewage-treatment/clariflocculator/
I agree and recommend the answer of Dr. Nabeel,and also to find flow depth, it is possible to use the book of " water works engineering", by the author "Syed R. Qasim" for the design of basin and other units of WTP.
Dear Dr. Ali hadi Ghawi, I totally agree with our colleague Dr. Isam for his definition of the depth of a Clariflocculator. The following link describes in detail the design information for water sedimentation tanks:-