The new Twitter design offers many new features which will enhance and simplify the browsing experience of yr timeline. However the new design also limits us slightly in the way we are able to customize it and create our own branding on our page.
I was searching for the new design specifications online, but found it hard to find any detailed measurements. So I decided to screenshot my newly migrated page and provide you with the basic specifications needed to customize your background and you avatar. Hoping you will find this helpful!
The new center column is twice the width (1020px) of the old design, leaving little room for a rich designed background. Find a template with guides created on a 1440 x 900 px screen resolution, using Firefox. Approximately with this screen resolution there is 190px of width on both sides of the page to be creative.
Download this .jpg in Photshop and turn on the guides (ctrl+;)
The new avatar image is much bigger (128 x 128 px), which is good news! However keep in mind that the smallest version of yr avatar is 24x24px so therefor I do not recommend to design it at the largest size. The timeline version is 48 x 48px and I would highly recommend using this as your reference size to design for. As aggregators such as Tweetdeck and mobile apps also use around this size to display you avatar.
I hope that this is helpful to you with creating your new branded page! :)
see mine at http://www.twitter.com/thomasmarzano