I’m working on a project to Android format 1024/600 (samsung tab2) when I put it on another smaller device will automatically take the screen size or something must be done?
I just created a thread with some helpfull pointers on how to make your gui as screen resolution independant as possible
http://forum.unity3d.com/threads/175774-Android-Input-misaligned-after-screen-resize-Screen-resolution-GUI-tips
The 3d part is based on the camera, which by default always covers the entire screen, no matter the resolution.
It might take some scripting to get the camera frustrum right (keep everything in view)
Check the documentation on camera tricks
http://docs.unity3d.com/Documentation/Manual/CameraTricks.html
I hope this helps, Good luck 
Oh, side note:
You can use the edtor “Game” view to quickly test different resolutions
Just change it from “Free Aspect” to another resolution and see what happens
Sorry for double post.