Help please. Should I resize first or last?

I am working with 8mb images that ultimately need to be increased to
50mb using PS bicubic. I am adding new layers like replacement skies
and flowers etc. and I need really good convincing edges. My query is,
for the best possible result, should I make my additions and changes at
the 8mb stage, or increase to 50mb and then make my changes there?
Many thanks for any help.
Richard
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richard