Lubuntu 18.04 LTS, Writer LO 6.1: This question has been asked numerous times but the answers address how to modify the footnote number font in the body of the document and not in the actual footnote. How do you do this? Update: I found the footnote option to modify on “format” then “page,” but there is still no option to tailor the number size. I tinkered with the “number height” and it had no affect whatsoever.
The style for the footnote number in the actual footnote is in a character style called Footnote Characters.
How could that bad description be more confusing? 
So in the Styles & Formatting side panel click on the “A” to get to the character styles.
Then modify the style called Footnote Characters and click Apply.
You will see the footnote number change style in the actual footnote.
If this has answered your question please check the checkmark at left.
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No idea how could one come to a conclusion like “the answers address how to modify the footnote number font in the body of the document and not in the actual footnote”, seeing answers like this. Even ignoring the built-in help system explaining that.
By the way, the question title (asking about “size within actual footnote”) contradicts the question body (explicitly asking that OP doesn’t want that, but instead needs to know about “number font in the body of the document”).
In general, the styles associated with footnotes/endnotes are assigned in Tools→Footnotes and Endnotes.... They are edited (including font size) by usual means all LibreOffice styles are edited.
I was pointing out “the answers” of others, insofar as they “address how to modify the footnote number font in the body of the document and not in the actual footnote.” Therefore you were hasty in declaring my statement “contradictory.” But thanks for giving me the answer! *When one is looking in the tools/footnoes menu, it’s by no means clear that ‘anchor number size’ (within the footnote) is adjustable. The terminology is nebulous. Would be better if simply said ‘anchor number size.’