To browse the BDA, you no longer require a username. That is only required if you are trying to download a file. The format has also been updated a lot. More to come soon.
Your old retroroms.net username will not work on the new site. You must use a username from the new site.
Limits on the BDA are not yet implemented. While the details are being sorted out, there are no restrictions on the BDA until further notice.