TL;DR: I disagree with the OP, and believe the current UI works for this site (that is, the Green Answer Accepted checkbox circle is fine); therefore, we do not need to be having everything aligning perfectly to a set size, nor do we need to even be discussing this issue further...
(This last part is obviously my own opinion, regarding this discussion not needing to go further)
I disagree with you, and believe there is nothing wrong.
Here's a zoomed in snippet, plus two black lines to show in-line size of the checkmark itself:
Here's what I see:
- The check box itself is slightly off-center from the rest of the buttons - that is, the checkmark itself.
- The green circle for an "Accepted" answer is slightly outside the size of these black lines... but only just so. I don't see an issue with this, namely because the green circle has spacing between its edge and the checkmark itself to not look crazy weird.
- The green circle is there for emphasis. It's done this way so people KNOW the answer is accepted (the green and white checkmark on the white background draws that emphasis).
This issue you've stated would only really apply if it were a case of the entire checkmark image itself being too huge - the checkmark itself is fine, the emphasis circle is slightly larger, but not such that it impacts UI.
Now, if this were on the Science Fiction and Fantasy site, then this would be an issue if the checkbox image itself were this way, and because I'm insane, here's a snippet from the Science Fiction and Fantasy site to give a comparison of why it would look really 'weird' in that case:
The SciFi page uses circular buttons anyways - which means it must flow in line there (the black lines are my own). Both of these are zoomed in. But if we zoomed out, we'd see a case of our green circle not really going far outside the lines anyways - thereby not causing a substantial UI impact.
My belief is the green circle fits fine into the current UI of Ask Ubuntu. We do not need to change it, and the slightly-larger-than-the-vote-buttons green answer-accepted circle is fine.