invoice img size

hey guys

I have been trying to create an invoice app, have created an img for this A4 size the same size as the pdf app.
Yet the img size is rediculas big.
So i copyied the image availble in the pdf invoice tutorial and tried the same, whee that image is stupidly small for any of the settings available.
Now tell me please what is the actual size of the pdf background to be if selecting A4???

As its not A4 size!!! So what is it???