The large flying bases (used for battleships - in this case read hive ship) are the big flying bases used for speeders/grav-tanks, whilst cruisers and frigates use the jetbike/gargoyle base, about the same size as a normal infantry base. They are in each case the standard flying base.
Could somebody please give me the approximate mini sizes of the various original nid ships.
Whatever looks good.
Since all measurement is base stem to base edge, or base edge to base edge, or whatever, the actual model sitting on the stem doesn't matter a hoot.
Size-wise...well, if you look at the metal miniatures
you get a pretty good idea of scale.
You can - with little effort - identify the plastic parts they've been cast up out of. For example the core of the 'Razorfiend' cruiser is a tyranid warrior torso - just like the warseer one. The escorts are gaunt torsos and/or bio-weapons - as with the metals (the 'Stalker' is built around the muzzle of a devourer), and the hive ship is in this case a 'fex torso - the GW hive ship is the only custom build, but that's about right for scale.The rules are availible free, but so is this article on custom-building bioships. Have a glance...
And to answer - most people use converted plastics for tyranid models - since the metals are casts of exactly the same thing, just more expensive!
Equally people tend to use converted plastics for dark eldar (the metals look awful) and forge world for tau (again, the metals look awful and the resins are so shiny), regardless of which tau fleet list they actually use.