This is a graphics and themes question. Fixing this will not require any knowledge of Simple Portal.
It is not that your problem is so difficult -- it is simply that there are not enough volunteers with any time at all on their hands to answer all the questions here.
It looks like you are using the default CURVE theme. In this case, you can solve the problem with a little CSS.
Using a tool like the IE F12 developers tool (which comes with IE) or Firebug for Firefox (a plugin), you should be able to find out what CSS classes you can use to style the "box" the logo is in. If you make that box bigger, a larger logo will fit.
Since you are not getting speedy answers here, I recommend a busier board -- like the themes and graphics board at
http://SimpleMachines.org/community. You'll find that many knowledgeable people hang out there.
I hope you won't mind that I've moved your topic to the Themes and Graphics board, where I think you might be more likely to attract more of the type of attention your issue needs.
Good Luck and have fun!