Ideal size for a windows

which is the correct size for a window that must see correctly in all resolutions? like a lightbox my code is:

var popupWin = Ti.UI.createWindow({
        backgroundColor : 'transparent',
        navBarHidden : true,
        id : 'popupWin',
    });
    popupWin.addEventListener("open", function() {
        this.activity.actionBar.hide();
    });
    //Event to close the popup window
    popupWin.addEventListener('click', function(e){
        if(e.source.id != null){
            popupWin.close();
        }
    });
    Ti.API.warn("device width "+Ti.Platform.displayCaps.platformWidth);
    popupView = Ti.UI.createView({
        backgroundColor : '#fbfbfb',
        height : Ti.UI.SIZE,
        width:Ti.Platform.displayCaps.platformWidth/2,
        borderColor:'#009fcd',
        layout:'vertical',
        borderWidth:1,
    }),