AH!
That's what I was missing!

I assumed it worked the same as .addRawSettings which just takes the base key name.
Though now I think on it, probably addRawSettings is just addRawConfiguration with "Settings\" prepended automatically or something.

Thanks for your help!

OK, but it's Logging\LogMaxSize and Logging\LogMaxCount.
I've added it to the documentation:
https://winscp.net/eng/docs/rawconfig

Ah right, thank you.

Unfortunately that doesn't have any effect either.
I set the following:
session.AddRawConfiguration("LogMaxSize","80000");
session.AddRawConfiguration("LogMaxCount", "5");

figuring LogMaxSize is in bytes by setting 1M in GUI and having it represented in the INI as a MB worth of bytes, I set it to 80,000 which is around 10KB.

And yet:
The file is sitting there at 141KB and counting, without having rotated.

Unless there's a validation somewhere in WinSCP that says the minimum value in the GUI (1MB) is the minimum viable value even when manually set (which I will test, but the file will take much longer to reach that size in the dev environment), I don't believe it's working.

For global preferences, you have to use Session.AddRawConfiguration not SessionOptions.AddRawSettings.
See https://winscp.net/eng/docs/library_session_addrawconfiguration

This should be in the .NET Assembly section, my apologies

I am using Session.SessionLogPath to set the log file to <fileName>.log in C:/WINSCP/logs/

I have tried setting the Max Size and Max Count settings in both the ini file and via sessionOptions.AddRawSettings, using a small value of 100 bytes or so to test immediately, and neither setting applies rotation to the log file. It just continues to grow.

I have even tried appending the Session name via <fileName>_!S but that just comes out as a literal _!S in the filename.

Am I missing something, or is it just not possible to rotate logs in this manner?