Since version 5.4.x.x this article has become a bit obsolete. The logo is now resized automatically to fit its container via max-width and max-height parameters of the classes listed below for different themes. So if you would want to increase the display size of the logo you would follow the article below and besides the steps listed there you would also change your values in max-width and max-height. Besides that you might need to do the changes also to .LALogoImage class.
What if your logo image is bigger than default?
In the Merchant panel navigate to Configuration --> Design --> [Affiliate panel theme OR Signup page theme OR Merchant panel theme ] > click 'Edit theme' at the current theme.
If it claims to create a 'Copy of .... theme', then click yes.
Now you can search for the "custom.css" file and place there the following code:
.bs-logo {
width: 230px; /* change this value to suit your logo width */
padding: 0px 0px 0px 0px; /* change this value to suit your logo position (top right bottom left) */
}
.bs-logo .bs-logo-link {
height: 65px; /* change this value to suit your logo height */
}
Then exit the theme editor and click the 'Set as default' button at the freshly created and edited theme. For more information regarding to working in templates visit the following article.