I was just pondering the other day as I was rendering a few images, obviously a nude figure will render faster than a fully clothed one... but I was wondering, does have all morphs injected into the figure affect the render process/speed/quality in anyway?

I've gotten some fairly big files with some of my figures.

I know the larger the file, the less memory there is to allocate for Poser to render, but is it significant enough to crash poser if I have too much for it to render?

Rule of thumb: The more polys in your scene, the more computing time Poser needs. Memory (mainly textures) and number of lights is another story.

To answer your question, yes.  I have had some renders bog my system down so much that it required a reboot to remedy the load.  It is best to eliminate as many props and figures from your scene that is not visible.  Reduce your dependence on too many lights.  For distant objects, use lo-res texturing, no need to see detail in those cases.  And if your actor is clothed, make those body parts that are hidden invisible.  Poser will not render objects it clearly does not see.
About morphed figures, keeping it simple will reduce the file size.  Keep the morphs important to your scene and eliminate the others.  All morphed characters should be saved in your library until needed.  Once introduced and posed, delete the extra baggage.

Morphs are just numbers, one set per vertex. In fact they are the deltas on your vertices, ie the numbers that need to be added to move vertices from their original positions to the positions in the morph (as the dial moves from 0 to 1)

The problem is Firefly only understands polys, it doesn't know anything about morphs or conforms or magnets or whatever, it just wants a plain mesh it can go to work on. So before poser can actually start a render, it has to take all those morphs etc and apply them to your mesh so that it's 'baked' into a form that can be sent to firefly. This of course takes CPU and RAM and if there's too many things going on, can really kill your scene. Even worse the process of baking is not multithreaded, so it only ever uses one core of your CPU which makes it even harder and more unstable.

This also jacks up your file size as with each morph, the list of numbers to be stored in the scene grows and grows, which of course takes longer for poser to read and arrange in your scene and to compress and write out when you save.