Logo size percentage?

When I look at my logo in my forums from my laptop, it looks fine because it fits perfectly to the width of the site itself. But when I come unto my sister's laptop (which has a smaller screen compared to mine) and access the forums, the logo exceeds the site boundaries and it looks really weird. Is there a way to control the percentage of the logo so that it fits inside the borders of the site no matter which computer is used to access the forum?

Thank you!

Use a smaller width of the banner.