text: edited Aug 28 '11 at 10:59
  The resolution and size of the viewer's screen along with their eyesight all have a major impact on that. I doubt you'll get a simple answer. – Cry Havok Aug 25 '11 at 17:12
  The answers are clearly not photography related either, but based on web site and general computer usage experience. (Thats not a criticism of the answers, or even the question - this just isn't photography related - it just happens to have the word 'image' in it.) – rfusca Aug 25 '11 at 18:48
  "Identification and Recognition Tutorial" from Axis Communications suggests that in surveillance, face width has to be about 17 pixels for recognition and 40 pixels for identification in ideal conditions (good lighting, face straight to camera); more pixels is needed when worse conditions have to be compensated by resolution. When accepting user-provided images, even more pixels should be reserved to account for head room in most portraits. – Imre Aug 25 '11 at 19:09
  Hi Jonathan. I'm going to try and find a better site for this question, and I'll move it there. Hopefully, on a more suitable site, you will get better answers than you have here. FYI, You may need to register on whatever site we move this to, but hopefully it will help you get the answers you need. – jrista Aug 25 '11 at 23:28
  answered Aug 28 '11 at 8:20
  There is more situations in which a Google+ profile image pops up. They go as small as 48x48 and even 24x24. You can elaborate a bit more on those cases to improve your answer. – Lode Aug 29 '11 at 6:23
  I would give people the opportunity to change the size, because it will vary based on the resolution and screen size. And then make sure that the image quality is reasonably high - 200x200 has been suggested, but you might have to experiment a little.
  The avatars for SO are too small at 32x32. However, I would probabl yjsut about recognise a face on one. It is remarkable how good we are at recognising faces that we know. The real issue is whterh they have to recognise the faces, or distinguish between faces. The requirement is different in each case.