Carrara’s Aura post-render effect is pretty bad. I *think* the number that you enter in its interface is the number of pixels that Aura is “spread” across (rather badly, because it always ends up looking very granular).
If you can see it in a low-res spot preview, but not in a full HD render, it *might* be simply because the spread is a very small part of the image at higher res… At anyrate, you will get MUCH better results if you use the Multi-pass function to render out a separate glow channel and use After Effects (or your NLE) to render the effect and composite the results over your main render.