Given the proper conditions, Cortaderia selloana 'Pumila' should reach 4-6' tall with plumes extending another 2' or so above the foliage. Very profuse flowering. Like other forms of pampas grass, the clump will continue to expand in size as it ages and won't need dividing until it starts to die out in the center, typically sometime after 5-7 years. Pampas grass is fully evergreen in my climate and is not routinely cut back each spring and since it is wickedly sharp, that's a good thing!! You can do some grooming and removal of old foliage as necessary, but I wouldn't bother to cut it back until absolutely necessary - I find cut-back pampas far more unattractive than other, deciduous/herbaceous grasses.