According to
https://wiki.mobileread.com/wiki/JetBook , the JetBook supports mobi and epub. It also says it has two embedded fonts and six sizes. I don't know if it supports custom fonts or not (ie you can load your own fonts on it). You could go to the Jetbook forum here and ask.
However, there should not be any problem with reflow and font sizes. These formats support this with no problem.