Help choosing coefficients for a simple FIR filter to extract DC component
MA is almost never a "most appropriate" algorithm (it's only advantage is simplicity). My advice is to choose Gaussian filter which has least possible group delay. Calculate its coefficients keeping in mind that cut-off frequency of a Gaussian filter is $$f_0 = \frac{1}{2 \pi \sigma}$$
Estimate which variances of low frequency component you'd like to preserve in your application, which are acceptable to be attenuated to 50%, this will give you desirable cut-off frequency.