Sizes for different OS and devices

Hi,

I wanted to develop an App for Android and iOS using QML and only one project. My problem is now that only looking at Android the app is displayed completely different on different devices. On one system boxes and fonts are quite small and on the next one the fonts go over the boxes etc.
So far all I found is that this could be related to the resolution of the screen. One suggestion is to calculate a dpi constant from the screen resolution and multiply everything with this. I tried that but this messed up everything on the emulator.

So what is the right way to cope with different resolutions, screen sizes etc. How should I start to build up an multi-platform app? How do I size fonts etc.? Is there something like a style I could use, so that I do not have to use absolut values?

Thanks for suggestions and/or links to good samples.