That URL has a user rank, not an avatar. Inspecting it did no good since it gives the classes for postbit userranks instead of postbit avatars. Without the ability to see the existing CSS there is no way to know what changes need to be made. The logical solution would be to use media queries on a "responsive" theme but that requires knowing the existing breakpoints of the theme. It does us no good to guess since we could end up doing more harm than good.
If it had been as simple as inspecting the existing CSS I would have posted a solution hours ago.
You are looking at the wrong thing (I did the same at first), the rank image is deceptively huge and looks like an avatar, if you go further down the page post 665 has a real one.