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There is no correct answer to this really. The 'perfect' width will depend on the column size your WordPress theme allows for the forum to display within. 

So - for example - on our forum here, using our 'Reboot' theme, the width of the 'user info' section to the left of all posts is styled to be 15% of the overall forum width (determined, as I say, by the width of the content area of your WP theme). This gives it a 'computed' width of nearly 104 pixels. This then gives us an optimal width for our special rank badges of 94px.

If the content width is smaller or bigger then the graphic will scale of course.

The best way to get the best answer s to find out the width you have available and work to that.  You can do this quite easily with your browsers web console which will allow you to look at each element on the page and determine the styling being applied. For a quick guide on how to do that, if you do not know, then this FAQ has a section on interrogating the CSS to get you started.