The registration page is typically set to full width by default. If your registration page is not full width then it may be due to the current theme.
You can adjust the width of your registration page in the Pages section of WordPress. This can be done by setting the page name WishList Member to full width.
Setting this page to full width should allow your registration page to appear at full width.
Please note that this page should not be deleted. The WishList Member page handles some behind the scenes functionality for WishList Member.