Opening popups can sometimes be quite tricky, especially if you need a specific size. This example shows a possible solution using the window.open function.

Using the following apex javascript standard-functionality at first, I wasn't able to let the popup have the dimension I wanted.

javascript: popupURL(...);

And I didn't want the user to scroll through the window either. That's why I used window.open with following parameters:

javascript: void window.open('f?p=...','popupwindowname','width=1000,height=650,status=yes,scrollbars=no,resizable=yes');

With that I was able to give the popup the correct size, so that the user doesn't need to bother with scrolling.

A nice documentation on all parameters and usage of window.open can be found at w3schools.com.