How do I use robocopy to list all files over a certain size recursively?

If you're not tied to robocopy, you can achieve this with using for's variable substitution:-

@for /f "tokens=*" %F in ('dir /s /b /a:-d c:\') do @(
    if %~zF geq 10485760 echo %F
)

dir /s /b /a:-d c:\ gives you a recursive listing (/s) of all non-directories (/a:-d) under c:\ in bare format (/b) for easier parsing.

for loops over that listing ("tokens=*" is needed in case you have paths with spaces in them), and let's you get the reference the file's size using its ~z variable modifier in any sub-commands (like if to compare against the size you want).

The @'s are to suppress echoing of the commands, and can be omitted if you've called @echo off previously (e.g., in a batch file).