There is a few modules which could turn the Drupal login form in a popup.
Drupal Forum – how to display user login in a popup?
Among those suggestions, i would prefer using the Modal forms module.
1. Download and enable the module.
2. Configure it @ admin/config/development/modal_forms.
3. Check it out.
Modal forms also allows developers to build the popup links manually. And the popup size could be modified in the module configuration page.
Done =)
Reference:
Eureka! 