You could take the 3 for £30 offer and you should be able to pay an extra £9 a month for the XL TV package. They should give you this choice when you ring up because it's the price difference between the L and XL TV package.
The latest Cable Set Top Box is the Samsung SMT 2110c:

The dimensions are: 310(W) x 225(D) x 53(H)
It isn't guaranteed you will get this box. You will get whatever box the engineer has in his van. The other boxes are slightly bigger.
Hope this helps.

to the forum BTW!