The TES Online team have answered several question relating to the upcoming beta on their website. They revealed that small groups of focused testing will occur during March and April.
Invitations to the general public for the first round of beta events will be sent out around the end of March. In addition, these beta events will initially be held on the weekends and each test will have a limited windows of access to the game. The beta itself will be around 20GB in size.
For full information, see elderscrollsonline.com