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).

Try including the %tmp% directory as the destination. I don't have the older variant of robocopy around to test anymore but I recall that /l actually lists what it is going to do not just the files. So you need to "send" the data somewhere it doesn't exist so that robocopy will print all the files and directories that don't already exist at the destination.