Previewgenerator - what resolutions are neccessary

I changed default settings with:

occ config:app:set …. squareSizes, widthSizes and heightSizes

What are the Default values - I don`t know and I wnat to reset.
What happens if the previews generation setting is too restrictive. Will it still create previews on demand then or will I get a bad display quality (hopefully faster)?

I found many files beeing created as preview but once I connect with my Smartphone there are still more file created on the fly and preview is still slow - despite the pre-generation.

How can I limit the generation and force the system to take what it finds?

Thank you.