Longer excerpts in LEVII?

Hi – I’m a WP newbie and am making my first site. So far I think I’ve worked it all out -using the free LEVII theme but there is one thing I can’t seem to figure out. Each post seems to be a fixed length which is quite short (like just one line of text followed by “READ MORE ….”). (see https://www.friday-fun.com/wordpress)

I’d like for posts to be longer – either fixed length but long enough for a video to be visible without having to press READ MORE. I don’t know css but poking around in the options it looks like I may have to add a bit of code to do that but really starting to get out of my depth.

Any pointers would be very welcome.
thanks
Mike