This is another of those kit bashed ships which are described in the DS9 Technical Manual. We've never seen it in an episode, for which I for one am quite grateful - it's hideous! Some fans have called this ship the "Medusa class", presumably because it is so ugly, but given the three nacelle configuration I think "Trident class" fits it rather well.

The dimensions assume that the saucer section is the same size as an Excelsior - I generally size all kit bashed ships in this way, since in terms of Treknology it is probably relatively easy to alter the length of a nacelle by adding or removing some warp coils, while re-sizing the saucer would require a much more extensive redesign.