MFCC window size at different sampling rates
I will answer your question from my experience. For most of the time, I've been using window length which is a power of 2, trying a few overlap percentages, training the system and picking the one which performs the best. 

In the case of 16000 Hz, I would try 256 samples (16 ms) and 512 samples (32 ms).
For 8000 Hz, my bet would be on 256 (32 ms). Don't forget to try a few overlap values, however, it shouldn't change much.

At the end of the day, what matters is the number of MFCC filter banks and the resultant amount of coefficients.