Screencast-o-matic is very user friendly. Powerpoint does have a recording feature, but the creator turns on audio record for each slide and then the viewer has to click on the speaker icon for each slide to listen. Very cumbersome.
The final file of any audio-video is going to be large. They can be compressed or reduced, if necessary.
Compress your files, and Keep the file size small!!
Divide lectures into different videos, "part 1," "part 2," for the topics you arecovering. Upload them to Google Classroom so that it automatically places them in the class Google Drive. It can handle large files.