One important consideration is small details.  If you have lots of repeated
lines such as the threads of cloth or shingles on a roof, a too-small render
will not only lose those details but often distort them.    You'll get false
indications that may look like a larger weave or an irregular pattern of
shingles.   (This is "aliasing".) 

No amount of enlarging in postwork will fix this distortion.

When you render "large enough", the details will be honest, and you
can later shrink the picture in Photoshop or Irfan without losing them.
The details may blur some, but they'll still be an accurate pattern.

How large is "large enough"?  You just have to start small and then try
larger until you see the details accurately.

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