Page dimensions

I' m tryng to customize the theme but it' s a little bit difficult due to that it works correctly only with image/phrases of the template.
In fact, I have to change background/header image and the phrases and it results impossible to have a correct position because if
I write something more or less longer than the contained phrase the result is different and the picture position changes continuously....
in which way is possible to fix the correct dimension of pages?
thank you for your cooperation

Hello,

Page dimensions you can change in John -> Template Options -> Layout tab, could you provide an URL to your website? (here or via PM), because I'm not sure where exactly the problem is.