Starting a new line with anywhere from 1-6 pound symbols in a row will adjust the font size, e.g. "##hello world" becomes
The more pound symbols, the smaller the font. You can see what the size will be in the preview window on the right (on desktop, no idea how mobile works).
I have no clue about strikethrough text though.
Edit: Actually I will put this here - https://commonmark.org/help/
It's a guide to markdown, a common simple text formatting standard used on many websites, including this one.