Below on the left is a 36-42" Natural Manzanita Branch set in plaster in an 8 inch pot, and on the right is an 18-24" Natural Manzanita Branch set in plaster in a 6 inch pot. In the middle is a standard wine bottle. Either would make a great centerpiece, but what size is the right size will depend on the size of the table and the look you're going for.
Keep in mind that the height of the branch doesn't necessarily have to determine the height of the display. In the picture below, we inverted an 8 inch flower pot just like the one the larger Manzanita branch is set in, covered it up with the fabric we use for our trademark burgundy backgrounds, and placed the smaller Manzanita centerpiece on top. In addition to creating a taller display with a smaller (and less expensive) branch, it also adds another element of visual interest.