General Asset sizes/screen sizing questions

I always get lost at the best screen size to use and best sizes for my assets. Kind of creating art assets in general.

How do I know what the right size is for my characters? My backgrounds? My tiles? My screen size? etc..

If anyone can shed some light on general guidelines that should be followed or typical sizes/dimensions when creating assets that would be awesome!

Thanks!

http://developer.android.com/guide/prac ... pport.html

theres a good read.with good links