I think the easiest way is to use the original logo size 245 x 75 pixels and just overwrite to old file with yours, calling it logo.png as well. That way you don’t have to change the coding any.
The file is in templates/ ja-opal/images/logo.png.
If you are using another color than the default you have to change the file in the folder for that color, like templates/ ja-opal/images/black/logo.png
If you want to change the size of the logo you have to change the template.css file in line 952 or so where it says:
h1.logo a {
width: 245px;
height: 75px;
display: block;
background: url(../images/logo.png) no-repeat center;
to something like
h1.logo a {
width: 450px;
height: 130px;
display: block;
background: url(../images/logo.png) no-repeat center;
which I have done on a site I am working on.
Good luck!