All of the more efficient reflectors have at least around a 24x24 lighted area. Some of the cheaper, less efficient ones may provide a bit of a tighter beam at the expense of total reflectivity. The narrowest reflectors I've dealt with are from many DE fixtures and lamps. IME, unless you raise some of these fixtures quite high, it won't be enough coverage even for about 18x18 (but at the same time, you compromise intensity). I'm not aware of any SE reflectors that will be as narrow as you want, though.