Probably the easiest way is to use firebug (in Firefox) or
the "web inspector" in safari or similar tools in other browsers.
If you right click on the info-box you will get pop-up menu with "inspect element" which will allow you to see the css tags controlling the image.
There is a width parameter which can be double clicked and adjusted (say set it back to 100% instead of 40%) and lets you see quickly what effect changes have. (there is a width setting which is probably the important one)
You can then make the changes permanently in your copy of conf/templates/math2/math2.css.
I don't know immediately whether that will have other side-effects. The current narrower percentage was an attempt to keep the info-box from taking over the top part of the screen when the window was narrower. --The 40% figure does that. it might be an over-reaction.
When you do svn update your local copy of math2.css will not be changed but will be marked M (for locally modified). Other aspects of the .css file will be updated if they are not close to your modifications.
You can use svn diff to determine what differences remain between your local copy and the svn repository copy.
For power users there is an excellent description of how to use svn at http://svnbook.red-bean.com/
I'm looking for ideas about how to organize "crowd sourcing" the updates to the .css files, and also spreading the word that one can create alternate themes by modifying the .css and template files to meet your needs. Then figuring out how to make these alternate themes available for the rest of the community.