What version of InDesign? InDesign CS6 and earlier can use a feature called Layout Adjustment. And in only CS6, you can use a more powerful method called Liquid Layout.
But neither method is totally automatic. Each method can require some preparation.
I am using CS5.
So can i just make the page to A3 & then the text & images automatically resize to fit the page?
Basically I am making some signs. The first document is 900x600 & I also need to do some A3 ones now in exact same format & everything.
Or you can place your 900x600 mm indd file to the new one (A3) and resize it as you wish. Any further text/layout changes (if needed) could be done in original 900x600 file, and A3 document should be updated.
Ability to place one indd file into another is a nice feature
And in the case where you want to maintain the layout and scale the text along with the page size change it's the method to use. Layout adjustment can do some useful things, if you are set up properly, but it resizes the text frame without cahnging the size of the text, so you'll either wind up with oversets or tiny text on your signs.