Mobile sizes are basically anything up to 768px wide.
The default theme settings are generally fine for any device of 320px or greater.
When you get smaller then that then you suffer the issue of too little space for the content.
You can do things like reduce the container padding ( or buttom padding for example ) on mobile to give it more room, or adjust the mobile font size for things like buttons.
But its best to design the site around your visitors – if you feel a lot of your visitors are using really tiny phones then for sure make those changes… if they’re not then i would leave be 🙂