I highly doubt that teachable.com is directly streaming your video. Rather, it will go through a process to prepare it for streaming similar to YouTube (at a high level). Thus, you should not constrain yourself to a low bitrate like this. You want to upload something high quality and without a lot of rate control - teachable.com will take care of that. I recommend that you try simply with the Export defaults (same as clicking Reset or the Default preset).
As for the resolution, if you are making a screencast some people may complain that your 1920x1080 screen in their web browser is too small looking. Some people screen cast at 1280x720 to make it more readable. It depends on what program you are recording too and whether it is traditional or modern web-like UI. Some of the more modern apps look fine recorded at 1920x1080 while something more traditional looking like Shotcut may not.