I have not got Portfolio loaded here in the office so it will have to be from memory so forgive me if things are not quite as accurate as they should be..
Look through youre templates css files for h1.logo. In there you will find some css along the lines of
h1.logo {
height:68px;
width:600px;
}
h1.logo a {
background:url("/~charter1/templates/ja_portfolio/images/logo.png") no-repeat scroll left center transparent;
display:block;
height:68px;
width:600px;
}
And adjust the width values as required.
If you cannot find the code download the excellent windows grep ( http://www.wingrep.com/ ) and use it to search your template folder. If you do a search of your template folder with it for the text h1.logo then it will find all files that contain this. You should have it sorted in just a couple of minutes.