What is the dimension of the TV stand when fully extended?

The fully extended dimension is 108.3"L x 15.7"W x 17.7"H.

Is there a matching coffee table?

The matching coffee table: https://www.homary.com/item/2-pieces-modern-wood-gray-black-round-nesting-coffee-table-for-living-room-35006.html

What needs to be put together?

It requires full assembly. Thus, all the necessary parts/pieces such as the top panel, bottom panel, back panel, side panels, legs, doors, shelves, and frame support need to be attached. The Installation Guide: https://img2.homary.com/mall/file/2021/10/11/8a3f57a0ae414221bbf3428ebe7f43bd.pdf