The Defiant is in-scale with the DST D and E, if you think of it as being 170m in length. Likewise, it's in-scale with the B (and Excelsior) if you think of it being 120m.
I believe I'm right in saying that the Equinox is also very close to being in-scale with the D and E, if it's considered to be 165m (the magazine said it was 220m which if true, renders it too small).
Obviously, the Klingon Bird of Prey is also in-scale with the D and E, although the length of that ship is anyone's guess! It looks 'right' next to the Defiant, so that's good enough for me.
These three are the closest in terms of scale, but they're not exact. Placed next to the big ones on a shelf though, and they do the job very convincingly.
Of the ships still yet to come out, I'd be willing to speculate (from those that I know well enough) that the Jem'hadar Fighter (Bug) will fit in well with the D and E (and Defiant, obviously), the Hideki class will be close to the B/Excelsior (150m) and good enough for me next to the Defiant, D and E, and possibly, the Oberth class will be close to those latter three as well.
One possibility I am excited about, and could work out well, is that the Runabout they make (Orinoco), might be close to the DST Bird of Prey.
EDIT: Got the Dauntless yesterday, and it's a stunner (forgot to mention).