Create your larger logo and upload it as logo.gif.
Then, EDIT http://www.metoxitusa.com/templates/ja_mesolite/css/template.css (or click on the template and EDIT and then EDIT CSS).
On about line 671, you will find this section (below are my settings):
h1.logo a
{
width: 180px; // CHANGE THIS LINE
display: block;
background-color: transparent;
background-image: url(../images/logo.gif);
background-repeat: no-repeat;
background-attachment: scroll;
background-position: 0% 0%;
position: absolute;
height: 100px; // CHANGE THIS IF NEEDED
top: 0pt;
left: 10px;
}
Edit the height and width to suit your needs to make room for your larger logo.