Use a text editor to open the style.css file in the folder that contains your copy of the default template set.
Scroll down the file, and locate the code beginning with:
/* IMAGE STUFF */
To adjust the size of logos, edit the following lines:
img.logo_image { width: 100%;
height: auto;
}
For example, if a logo looks large on the setup sheet, change the width setting to 75% (the aspect ratio is maintained because the height is set to Auto).