If you want to create a fixed size popup window in Dreamweaver you can do so by assigning a behavior. Make sure your Behaviors Window is open and the text, image, or area you want to assign the behavior to is selected.
In the Behaviors window click the + and select "Open Browser Window". Assign the parameters you want to the behavior and click ok. Make sure you adjust the trigger action to what you want (i.e. onClick).
Abraham