Sometimes it seems like fans are under the impression that if you use a saucer or nacelles that have the same physical appearance as those used on the Enterprise or Miranda, those parts must be the same scale. And that is not necessarily true.
It does create some pretty big problems if one tries to use a
Constitution saucer to depict something smaller - the windows become even more nonsensical than before, and most of the saucer becomes way too thin to provide any sort of inhabitable volume.
And it would seem a major engineering challenge to alter the size of a design without altering its shape. There's a good reason why elephants don't look like giant ants.
In case of the
Jenol*n, it's rather clear anyway that she's supposed to be
big. Rows upon rows of windows, a bridge dome, a big shuttlebay with a control booth above the door, plus the fact that she seems to be at least as big as the E-D when blocking the Dyson Sphere doors...
It was supposed to be warp 4, but the real answer was "as fast as the story required".
In "Dax", it was a plot point that ships capable of warp 5 or above would be viable escape vehicles for the villains, heavily suggesting that runabouts were slightly slower than that. Warp 4.7 was quoted in some background material but never on screen.
In "Jem'Hadar", the panicking Nog and Jake tried to make the runabout go warp eight. Doesn't mean the craft would be capable of that, though.
In some later episodes like "Treachery, Faith and the Great River", runabouts seem to be able to give credible chase to / try to escape from Jem'Hadar bugships that in other contexts are the propulsive match of the warp 9ish
Defiant. But this is no definite proof of any speed improvements. Warp 4.7 sounds nice enough to me - 0.7 warp factors higher than the highest known shuttlecraft speed (the one quoted for the unseen Type 9 design in VOY "Resolutions"), and reasonably higher than the speeds usually quoted for civilian shipping.
Timo Saloniemi