Heading sizes?

If using 2 or 3 different sized headers in a Kindle book, what sizes would you recommend I use (or not use).

Suppose I opt for a large sized heading (h1), could that cause problems if someone is reading it on a smaller device - would the headings reduce in-line with the paragraph font, or would they stay huge, and fill the whole screen?

Or should I not worry about this? I guess most people will be reading them on e-readers that are made for ebooks!

All help is appreciated!

Thanks in advance - Zoe