Quote - When I change the render dimension instead of rendering what is in the preview window it moves around and generally crops the image to suit itself.

This happens because of the aspect ratio. If the aspect ratio is different from what is in the preview window, it will crop the image with black bars. What you wanna do is pretty much everything alanadale said. Go to "render dimensions" and select "render to exact resolution" and click the "match document window" This will make sure the ratios match. The default is 640 pixels for width. Increase this to however big you want your render to be, and now the height will automatically adjust. You can render large images without those nasty black bars now.

Quote - There is a slider for up and down but not one from side to side when I view the popped out preview. Its never exactly how I wanted it and I'm sure its me doing something wrong here.

When i pop out the window, i usually resize it using the bottom right corner. This way you can change it's size both horizontally and vertically. To get the exact size you want, select "window" on the top of your interface, then select "document window size" Enter the width and height here. If you want your aspect ratio to be something like 4:3, then enter 600 for width and 450 for height. When you render, do everything above to maintain the same aspect ratio but get a larger render.