Is there a rule of thumb for zero padding in image processing?
See Press et al. "Numerical Recipes in C++". Chapter 13 on "Fourier and Spectral Applications", Section 13.1 with a subsection entitled "Treatment of end effects by zero padding".  This is perhaps the best summary of zero-padding anywhere in the literature (but it's not referenced in either the contents or index).  My edition of this book is the 2002 second edition and the page number there is 545.