[Shelimov]

Как вариант:

<a href="/blabla/" target="_blank">Link</a>

document.getElementsByTagName('a')[0].onmousedown = function() {
    
    var scr_wdt = screen.width,
        scr_hgt = screen.height,
        win_wdt = (scr_wdt / 2) + 300,
        win_hgt = (scr_hgt / 2) + 200;
    
    this.href += "?width=" + win_wdt + "&height=" + win_hgt; 
}

Или второй вариант:

<a href='/calc/index.php?step=1&TB_iframe=true&height=%(my_param_x)%&width=%(my_param_y)%' onmousedown='work();' rel='sexylightbox'>

function work() {
    var scr_wdt = screen.width,
        scr_hgt = screen.height,
        win_wdt = (scr_wdt / 2) + 300,
        win_hgt = (scr_hgt / 2) + 200,
        my_params = {
            my_param_x: win_wdt,
            my_param_y: win_hgt
        }, param_name;
    
    for (param_name in my_params) {
        this.href = this.href.replace('%(' + param_name + ')%', my_params[param_name]);   
    };
       
}