The largest aircraft ive seen in Dundee is the BAe146 when it used to operate the Palma flight, albeit this had a touch down in SEN or BOH i seem to recall.
Im guessing something like a Do328 or Saab 340,would be an ideal candidate for reopening the BHX route, possible alongside a MAN flight, maybe a twice weekly Belfast too. Saying that there is talk again of RAF Leuchars opening up a bit for commercial traffic again, my mate live in Leuchars.