When I was a kid building Airfix models and the like, one of the rules I remember of painting was that the shiny surfaces of large objects (ships, aeroplanes) should not be painted gloss.

So I'm wondering how this applies if at all to rendering, and whether I can rely on the rendering engine to deal with the scale issue. Blurring reflections is a possible approach, but render times on reflective spacecraft is already killing my computer as it is!!

Any thoughts on this?

If you don't care about reflections and just want a reflective look, you can always use reflection maps instead of a full reflective material.

Depending on your scene, that could do the trick for large surfaces.

Blurred reflection increase render times a lot. One easy substitute is a very fine bump. Not exactly the same effect but close enough for most purposes and much easier on the render engine.

Why don't you find a reflection map you want, blur it in photoshop and then apply that to your object.  There you have blurred reflection without the wait.  Try doing a small render of your scene and apply that as your reflection map after you blur it.

 The problem isn't the reflective material, but the perfectness of the surface. Think of those reflective office buildings one oftens sees in skyline pics. Because the glass surface of the building isn't perfect, the reflection becomes distorted somewhat. It is very difficult to create a perfectly shaped large object.

Adding a slight displacement to the model, or as Rutra suggests, a small bump map might help.