I dont know how to create a navigation taht is exactly 900 px width and 50 pix height.
the text of the navigation buttons should be horizontal and vertical centered.
HOW DOES THAT work.
CSS offers you many many ways of doing so...
For instance, you can encapsulate your CSS menu in a DIV tag which is the width and height you want with the same background as your menu.
You can even position the entire menu with padding or margin...
You can set the menuitems in the middle by manipulating line height, padding etc. etc. align the item to center and done. But there are many other ways...
To keep your pages compatible in all browsers use the KISS principle : Keep It Simple and Stupid.
You can not expect that a tool does all the coding for you, it's best to learn HTML and CSS and then you can use the tools much better. And yes, creating websites is all about testing testing and debugging.