If you are using the child theme then it is very easy to do without hacking the core!
Step 1: create a logo of the correct size/dimensions so that it doesn't get distorted when you resize it using CSS.
2) Now in your style.css file enter this code or something similar to suit your image size:
/*Header image and custom size */
#branding img {
width: 1000px;
height: 130px;
}
3) Make sure the width is always 1000px because that is the default to fit within the page wrapper. Height can be anything to suit your design.
Please do all this changes in your child theme so that you don't mess around with the original theme.
This works because I have done it here:
http://mytechhelp.co.uk/
Hope this helps.