I think, that you can answer the question yourself. Besides frequency decimation, that can be achieved by a first order CIC, the main purpose of a decimation filter is attenuation of aliasing signal components. By defining a cut-off frequency and a stop band attenuation, you get the required filter order. It's discussed in Hogenauers classical paper An economical class of digital filters for decimation and interpolation and many text books, e.g. U. Meyer-Baese DSP with FPGA.