This is not possible in Syncthing without some hackery*. There was a very heated discussion on this particular topic in https://forum.syncthing.net/t/cannot-find-max-file-size-in-documentation-and-some-other-specs/17286 recently. You may want to read it thoroughly.
*Unless you run multiple instances of Syncthing simultaneously. You still won’t be able to sync the same folder at the same time, but you can sync different ones this way. Please keep in mind that doing so will use more resources and may require some babysitting to keep everything separate. The Docs have the required information on how to use command line switches to use separate configs and such, and you will also find plenty of information on the forums if you search for “multiple instances”.