Zinga is da best...

well for making themes there is no exact science , you get the rcos and start editing ...
abd for avoiding memory problem (missing icons , whitye squares) you must not use large icons
use exactly the dimensions you need for the icon only.

For making things bigger , search in rco editor in page data of the rco , strech , and try to change to something
more than 1.0 is the normal , you try something above that but below 2.0 because your icon might become pixelated.

i hope you understand me.