You cannot control this - it's entirely at the discretion of the user-agent; which is the point, after all. All you can specify is that the page be opened in a different viewpane context, and it's up to the user to decide how they want your window to take up their screen space/taskbar list/Alt-Tab shortcuts etc.
In fact I'd go even further and say that if at all possible you should avoid opening up a new tab/window at all. I know that I get a little annoyed when websites do this, and it feels a bit clunky and 1990s what with all the Ajax and floating divs and magic we have nowadays.