You should always position HUD/GUI/etc in relative terms (for e.g. relative to the edge of the screen) to make your game aspect-ratio-independent.
As for different sizes, this is a design
issue. Non-artists and certain programmers might just want to scale the game's assets, but this is a poor design choice. "It’s simply not possible to create excellent, detailed icons which can be arbitrarily scaled to very small dimensions while preserving clarity. Small icons are caricatures: they exaggerate some features, drop others, and align shapes to a sharp grid."All the sizes of iOS app icons
Simply scaling down:
Better practice, re-designing the assets for different target sizes:
Another thing to consider: do you want to support full-screen on desktop?
- If you don't care, then just design your game with a fixed size. The differences between Android/desktop will be minimal.
- If you do
want full-screen support, you'll likely need a significant
redesign of your game/assets/HUD/layout/etc.
For the latter, consider how web designers strip a desktop website down to a mobile website. They take away superfluous elements, emphasize certain aspects of it over others, etc. You might be able to learn something from these articles:http://mobiforge.com/designing/story/effective-design-multiple-screen-sizeshttp://uxmag.com/articles/framework-for-designing-for-multiple-deviceshttp://www.smashingmagazine.com/2009/01/13/mobile-web-design-trends-2009/