There are two part of an affiliate sign up form: the left side with smaller width is for your brand name and the program sub main heading, the right side is the main part which contain logo, program main heading as well program details and fields. You can customize to hide the sidebar in specific case following these steps:
1. Go to Settings > Affiliate registration;
2. Click Custom CSS tab and add the following code:
3. Click Save changes to update.
Then the Affiliate registration form would look like this: