The current default window size for estimating coherence is common for both range and azimuth (10x10 pixels). Since the size of the pixels in SAR geometry are not the same this is causing visually elongated pixels when projecting/geocoding coherence maps into map geometry.
To compensate for such effect the window size for coherence estimation should be accounting for the difference in range and azimuth posting, similarly to the multi-looking operator. I would recommend a 27x7 window for range and azimuth, respectively. In that case the output in SAR geometry would appear having elongated pixels, but the geocoded results (square pixels) should be sufficiently better.
The window size for coherence level estimation should not affect the size/resolution of the input/output products.