[Theme Horse Support Team]

You have to edit the logo with adding some gap around and upload and yes logo should be about 400px (width) and 150px (height).
Though if you just want to add some more space between the logo that you have right now then just Go to Appearance -> Theme Options -> Design Options Tab -> Custom CSS paste the following CSS code and Click on save all changes button:

.widget_ourclients ul li {
    margin: 0 30px;
}

Regards,
Theme Horse